You obviously don't know RMS' views very well. Read him in his own words (look for the word "alternative"). Before you take exception to the alternate career suggested, here is what the first employee of the FSF had to say on it. And then explained why it was a particularly good option to consider. (For the record, Brian built a company up to reasonable size and, with wealth taken care of for some time to come, stepped down to have more time with family.)

Whether Richard has his head up his backside is your judgement call. Many people think that. Many others think that he should be (atheism notwithstanding) canonized. I say that he has a carefully thought-out world view which is unusually consistent and inconvenient to a lot of people. If you don't share his assumptions, then the reasoning process is interesting, but the conclusions can be disregarded. Adjust this advice to the extent that you share his assumptions and find the reasoning compelling.

That said, my advice to the original poster is to accept the job if it interests you. It is good experience, and more importantly it is a place to get skills from which you can get another job. This is not just, "It looks good in the resume" - it looks good because you actually improve in non-trivial ways. Don't underestimate that.