If the majority of customers prefers another beer they also sell? Yes, kind of. If the customers most likely wants the other beer, you're wasting his time by asking him whether he wants to try your special brew.
I don't believe Microsoft is attached to making Windows the default though. Who knows why it is, maybe because people that use the Wizard are likely playing around and will rather want a Windows system? My guess is they'll switch the default if the percentages shift even more and they see that most that manually set something up will change the default. In the end, they want to sell computing and anything that gets in the way, well, gets in the way.
There are tons of restaurants that have their own specialty drinks like their home made soda or root beer or their own alcoholic beers, but most customers pick Budweiser and Coca Cola.
It's not crazy for them to suggest their own home-made soda first, it's marketing. Because that specialty soda makes them more money and differentiates them from their competitors who are just selling off-the-shelf stuff.
If you're trying to serve a mass market? I think it is. Wasting time (yours + the customer's) by making them go through and deny the less common options isn't a good idea if you're aiming for serving as many people as possible as efficiently as possible. This would be very different if it wasn't Azure but some local tech company that sits down with you to talk about what kind of server you need and want, what OS best fits your requirements etc, but that's a very different game from what Microsoft is playing with Azure.
With low/self-service, streamlining the process is the right thing to do. In the end, you have to ask yourself, whether you want to earn a few bucks by also selling a licence to your OS, or whether you want more customers. Of course, it's not an actual issue in this case, because you'll usually not manually provision (but then again: why do it if you don't expect to make money by doing it?)
They keep doing it because it works. Same reason restaurants tell you the specials when you first get to your table even though you’re going to end up ordering a burger anyway.
You can argue all you want about what they “should” be doing but they wouldn’t be doing it if it didn’t work.
