I think some people (including the other repliers) don't quite understand the GPL. You don't have to distribute the source code, but if you distribute a binary, you must offer to distribute the source to the people you distributed the binary to. Similarly, you can't stop them from redistributing either the binaries or source, or modifying them under the GPL license.
So charging for GPL software (whether binary or source) is far from useless. Back in the day I often bought CDs with a Linux distribution on it. It was easier than downloading it when the internet was not as pervasive as it is now.
Having said that, I don't think they have a viable business model. Charging for distributing a Linux base OS, even if you have added your own changes, is unlikely to make a lot of money. They would probably be better off trying to find a niche and then do contract work to improve it.
Someone could put the code on github and have an s3 download link up in all of 5 minutes. A blog post and 15 minutes of retweets later and it has the number 1 spot on Google for elementary os.
It's incredibly trivial to spread gpl code now so charging for it is a non starter and just makes you look a bit foolish.
A large portion of the appeal of something like ElementaryOS is going to be that I do not need to download source code, build binaries, try to follow some poorly-written installation README file, etc. I would certainly be willing to pay $10 or more for the convenience of not having to do that. Focus on your paying customers, and just accept that some people will not pay. So what? If your code ends up on github it isn't costing you anything, and you're not going to lose many paying customers because the people going to github were never going to pay you in the first place.
You definitely can do that, but most people will not. As I said, I don't think they have a viable business model, but that doesn't mean that it is impossible to make money charging for GPL code. As an example, take a look at the game Tales of Maj'Eyal. You can download it for free on te4.org. You can also donate money from that website. You can also buy it on Steam and a few other places. It was also available in a recent Humble Bundle. People have paid for it and continue to do so even though it is well known that it is available for free at the author's site. If the forum is anything to go by, people who buy it on Steam just find it easier to use Steam and are happy to pay a few dollars for the convenience. You can play the game online which gives you a few benefits. The author has reported in a recent Roguelike Radio interview that 70% of the people who play online have either donated or bought the game. (Admittedly I do not know if the author makes enough money to survive only on sales of his game alone).
Having said all that, you are correct that you have no vendor lock in and people are able to change suppliers (including no charge suppliers) without any cost to themselves. As a supplier of GPL software, you can not rely on artificial scarcity to pay the bills. There must be some other reason for people to pay you (not the software itself). In the case of TOME, people are paying for the convenience of using Steam, or they are paying as a way of thanking the author.
There are other niches where you can make money selling GPL software. For example, quite a while ago I worked for a very small start up that was trying to build a VOIP infrastructure company. The idea was to enable new, small ITSPs to get up an running. We bundled and sold turnkey systems that was composed of free software. None of our customers cared that they could download the software for free somewhere else. They wanted a turnkey system that was configured and set up for them. They also wanted us to support their infrastructure so that they could concentrate on sales. The cost to us for packaging everything and doing custom configuration was dramatically lower than it would have been for them to try to set everything up themselves. Often they didn't even have a technical person on their staff. We could charge for our time and also charge for the software and it was still quite a bit cheaper for our customers to use our services. (As a side note, we unfortunately moved out of this lucrative area to concentrate on selling proprietary software without attached services. This ultimately failed, which I have often thought is a bit ironic.)
The thing is, I think I understand what you are trying to say, but you are making too general a statement. There are lots of ways to make money selling GPL software. However, you can't sell it the same way you would sell proprietary software which is why this company will probably not succeed unless they change tactics.
So charging for GPL software (whether binary or source) is far from useless. Back in the day I often bought CDs with a Linux distribution on it. It was easier than downloading it when the internet was not as pervasive as it is now.
Having said that, I don't think they have a viable business model. Charging for distributing a Linux base OS, even if you have added your own changes, is unlikely to make a lot of money. They would probably be better off trying to find a niche and then do contract work to improve it.