It's simple to answer why they are able to pull this off: There's a law for that, a law that dates back to the introduction of tape drives. Back then, there were two options:
1. Allow copying for private purposes, introduce a flat fee on empty media (tapes back then, later cassettes, cd-roms, ...)
2. Ban all private copying, prosecute it at all costs. Ban tape drives, control all technology that can be used for private copying.
3. Copy as copy can, no one gets paid. Ignore that this may be an issue for people that depend on producing (music, films, ...)
The (IMHO wise) decision was to stick with option (1), so that's where we're at. The GEMA and ZPÜ, as much hate is directed at them and as much as is wrong with them, are tasked with collection that fee. It's their job and they do it, even if it's unpopular. It's a bit like the Finanzamt (IRS). You don't have to like them, but at least keep in mind that they're doing what they have to.
Now, one may argue that the current system is not suitable for the digital age where empty media is used for so much more than copying audio, or even that the whole GEMA system should be abandoned - and I'm actually on your side in that respect - but well, that's a different topic.
You could also appeal to the regulatory body and argue that the price is too high, and that's what the bitcom rightfully does, but then you'd have to settle on an acceptable price. So what's acceptable? I don't have an answer, do you? They'll eventually figure this out and the high prices demanded are part of their negotiation strategy.