The petition only makes legal sense if it were to ask to extend the set of charitable goals as specified in the Abgabenordnung, but the existing set already allows for FOSS projects as part of e.g. the "national education" category (public code is educative).
And, to be frank, I also don't get the "recognition" part. The tangible benefits of volunteering for a charity are limited; what does it even mean to get recognition for it.
It may be educative but that is hardly the most significant way in which open source code is beneficial to society.