Then have to design and print t-shirts, manage inventory and so on, track earnings, etc. Even when you can outsource that, it might not be worth the headspace for them.
Shell out credit card, get email.
They had a public campaign asking the public to "buy money", selling a hundred Euros (and two post cards) for the small price of exactly a hundred and five Euros -- effectively selling postcards for €2.50 a piece with a transaction overhead of another €100 in expenses and income.
Due to the way German public funding of political parties works (the public funding was based on revenue rather than profits) this meant they were granted funding that was hugely disproportional to the actual profits. Why it's supposed to be based on revenue or profit in the first place I don't fully understand but let's just say the authorities were displeased.
However they have a good case: they did the entire scam out in the open and had legal counsel that greenlit the campaign. It's definitely violating the spirit of the law but they may be able to make a good case that they were trying to showcase the absurdity of the law rather than simply exploit it for their own benefit -- also nobody interfered at any point and they documented everything from start to finish.
If they lose in court, however, not only would they have to pay back the money they were given (€72k) but they would also have to pay a massive fine (€384k) which would bankrupt the party instantly. Considering the party's satirical nature this would be pretty disappointing.
I might start doing that...
Something I've seen on other sites, like GameDev.net, is that you can buy a membership, which for all intents and purposes is a donation, since it doesn't really get you much besides an extra tag next to your name.
For a while a popular way around PayPal's restrictions was for sites selling services that were in violation of PayPal's ToS to go through an intermediary site for donations/credits. So you would buy a crappy e-book for $20 or so, or maybe a couple of mp3s for $5 each and forward the receipt to have the equivalent credits added to your account.
Pretty clever as the ecommerce sites themselves had full plausible deniability -- there was no obvious link between the PayPal merchants and the (most likely anonymous) people running the service.
I don't know how successful it is from a financial perspective, but it seems like a model that could be applied to almost any community/discussion oriented sites.