You also can just see the difference between your potential salary and what you get as your personal contribution to open source. And lots of people are happy to receive something, even if the value of the gift is not significant.
Some experienced, well trained, 1st world developer is not satisfied with $300 for 5 minutes of his oh-so-superior work on FOSS software? Please excuse me, while I try to parse your statement in some nice way...
I think their point is that there are a lot of people for whom $300 is a great payment - essentially, defending the maintainers for offering what the parent seems to be implying is too little.
Personally, I'm a senior dev making more than triple what I did when I started my career, and if it were five minutes of work (or even ten, or an hour) I'd be extremely happy with $300.
I included people that are doing FOSS work in there. I also have found very few software engineering tasks that are 5 minutes of work. I am not looking down on 3rd world developers, it is just a simple fact that in most of the western world, doing a few days of work for $300 is not going to equate with getting a job as a software developer. Assuming it takes a week to tackle one of these bounties, 40 hours of work, That is a bit above minimum wage in the US. Even 20 hours is only $15 an hour. I don't think this will inspire professional developers in expensive nations to try and make a serious effort, for just the money for just a simple fact that it doesn't pay enough. But there are many classes of developers that this is good money. I am defending this bounty.
undergrad or grad student looking for extra cash.
dev in a third world country where that might be an average monthly salary.
junior building up their resume and trying to pay off some student loans.
someone that is going to be doing work anyways and at least gets something.