To many people, Ubuntu represents nothing but a shiny front end build on top of the years of hard work spent on building Debian, the universal OS.
These people (we) are willing to use Ubuntu, since it is pretty much the same thing, but they (we) will never go out and actively support the Ubuntu project as we believe that Debian is more deserving of our support.
I think I (sorta) understand your reasoning, but it doesn't make much sense to me. When I write some application that makes it easier for my clients to do some previously complex tasks and I ask them to tip me, I don't expect them to tell me that the previous system or database did most of what made my work possible.
These "donations" aren't just there to thank developers for past works, but also to equip the distro so that it can improve and continue to push the envelope its way.
Ubuntu's users chose to download and install Ubuntu, not Debian, probably because they like whatever Ubuntu does that you find so insignificant and that other distros aren't doing. I suspect that you too are running Ubuntu, not Debian, for the same reasons.
When Ubuntu asks for money, that's because Ubuntu needs it to keep doing those insignificant Ubuntu stuff, that made you choose to install it, not Debian. So, don't bring up Debian.
When Linux Mint started their own distro, they didn't go all the way down to Debian. Ubuntu probably did something worthwhile, that Mint would rather not do themselves. But people who installed Mint didn't install Ubuntu, so when Mint asks for donations, it's for Mint stuff. Let's not bring up Ubuntu.
While all donations are welcome, it is especially hoped that any businesses that make money through Debian (CD manufacturers, support companies, or even businesses that rely on Debian for day to day operations) will contribute a percentage of their profit to help make Debian the best OS it can be.
Get it? help make Debian the best OS it can be. Let's not bring up GNU or Linux.
Well, you could support both, apportioned however seems reasonable to you ... e.g. "Ok, Debian provide the bulk of the core system, so $50 to Debian, and Ubuntu does some good work tweaking the UI, adding shiny, and holding users' hands, so $5 to them."
These people (we) are willing to use Ubuntu, since it is pretty much the same thing, but they (we) will never go out and actively support the Ubuntu project as we believe that Debian is more deserving of our support.