For example, if you do volunteer work in The Netherlands you can get at most €5.60/hour, with a maximum of €210/month and €2100/year. I assume Germany will have similar rules.
€12/hour is just about minimum wage. Explaining how that isn't a salary is going to be pretty much impossible - it'll rightfully be interpreted as tax fraud. On top of a violation of labor laws for paying less than minimum wage, of course.
I do see a lot of benefits, though. There are plenty of people who aren't well-off who are doing incredibly valuable work for F/LOSS project. If you're holding a conference you really want to be able to invite those people without putting the burden of travel expenses on them: a €200 train ticket can easily be a dealbreaker for a poor student.