He's Belgian. Belgian taxes are awful. If you want to accept 'donations' for software, you have to be self-employed. This comes with a boatload of hassle, including paying national insurance contributions of 20%. (which are calculated based on your income from three years ago). And given that the man has a day job, he'd also be looking at a 50% tax rate for these donations. So a 100eur donation would net about 40eur…
I'd maybe look into it at $1000+, but most bounties aren't really anything close to making it worth your while if you have a job.