"don't theme my app" is not about users theming their apps, but instead against distros theming by default and more importantly the expectation that you could theme every app with a generic theme.
I suspect even their attitude towards theming done by end-users is more akin to ‘well sure, if you insist, you have the full source code, I can’t stop you, but you’re on your own’ than considering it a fully supported configuration.
There can also be legitimate reasons not to support it. On the top of my head, for example, the default theme being the most tested, etc., etc. In a perfect world, they'd all be tested and well integrated, but there's limited man power going around, and the world isn't perfect.
Though I otherwise agree.
Maybe distros are not doing a good job of this, but that is one thing they have the power to do. QT, GTK2, SDL, wxwidgets, and more toolkits I can't even think of should all fit together on my desktop and look the same. xfce, gnome, and kde all have some nice apps, they should mix and match. This is a HARD problem, but that isn't an excuse to not face it.
I would reject a distro that did this (unless I could disable it). But users should be able to make everything look the way that works the best for them.
We could shift cultural expectations so that distro maintainers become the first tier of support for all app issues in their distro, and escalate issues to app developers if and only if they've confirmed that the issue is distro-agnostic. That would be sustainable for app developers but could be a heavy burden for small distro teams.
> This is a HARD problem, but that isn't an excuse to not face it.
Agreed, I just don't think it's the responsibility of app developers to solve.