Think of it on the flipside. If I make a website, I don't control who accesses it, and if I run ads or something, figuring out where that revenue is coming from is quite difficult. It can be done, but if I have to pay taxes to a hundred different countries, that's quite the burden.
I don't know how DSTs work in practice, but ideally we'd just discourage ads in general. Paying taxes on actual transactions (sub fees and whatnot) is easy, and ads suck.