At least it should be. I'm not sure what definition this petition would use.
In fact I see no reason why you can't already get this recognition in the existing legal framework by creating an association with a specific scope.
it's a petition, not a law proposal
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.
Hell, most FOSS today was created by a single individual/organization for themselves, figured it might be useful for others so they publish it under some FOSS-compatible license. That then others found it useful is the cherry on the top, not the core motivation.
(As a donor I tried to sell blood when I couldn't afford rent and food, but it seemed impossible.)
I probably do hundreds of thousands of dollars' worth of work, for free. It would be nice to be able to use some of that, as a tax break. It would be a drop in the bucket, compared to the monster breaks corporations get.
I would be more worried about rigidly defining "open source." We see battles on HN, about the definition of "open source." It could end up specifying something like only release of GPL-licensed code is allowed, which might seem OK, but that's sort of taking a "political" stance.
I release all my stuff MIT (usually). That's mainly because I don't care if anyone takes it, and gets rich (Fat chance, anyway. My stuff isn't that amazing), and I'm not interested in coercing anyone else to take my political stance. I just don't want some bunghole suing me for something out of my control.
So it should be the FSF's definition of free software, https://www.gnu.org/philosophy/free-sw.html