Of course, if people are actually willing to pay for GPL'ed software, even if the money does not go to the original author(s) and they could just as well get a legal gratis copy of it, then I guess that's their loss. I never thought people would be prepared to waste their money like that on a significantly large scale, but I guess the iOS and Android ecosystems excel at attracting sufficiently stupid users who are always ready to throw away their money in $1.99 increments -- after all, taking a percentage off of that shitstream of microtransactions has become the new business model of Apple and Google.
That's a pretty condescending attitude you've got there. Paying for software instead of grabbing the source and compiling it yourself doesn't make you stupid. Even if you're aware that it's GPL, compiling it and making the binaries available is still a service that many people aren't capable of (and no, not knowing how to compile something for Android doesn't make you stupid, it makes you ignorant of a pretty specialized skill same as most people are with most skills). This isn't just doing it once but potentially many times, where it then shows up as an automatic update. You may also get some level of support meaning you're more likely to get someone to help you figure out what's wrong when it doesn't work on your device - given the mess of hardware/software configurations on Android (that far outstrip what you'll see on say, Windows) this is an inevitability.
And porting to Android is still a decent amount of work in all but the most trivial cases and some people will want to voluntarily support this.
Bottom line is, even if I knew I could get the GPL source and build it myself I'd probably still fork over the few bucks because it's just not worth the hassle.
Maybe you're really blaming all the people who could be porting it and maintaining/supporting it on Android for free but aren't, but if that's really your stance you're probably going to have to add yourself to that group..