Especially if you keep some money for yourself, it's a good deal. Setting up the account is a minor inconvenience, so a disproportionate number of people are not going to do anything about this. Taking a cut for yourself allows you to take whatever cut you want to overcome said inconvenience: if you think it's about 10 dollars annoying, you keep 10 dollars, and then 40 (80) goes to SIAI.
The compensation cancels out with the annoyance, and you're left with fuzzies.
That is why I praised and upvoted it, but I don't think it's what User:Kevin intended. I think he intended to answer your question
why should I extend them $50 of credit for 50 days
with "you get $2.50". I believe this intended meaning is obvious because he quoted your question and provided an answer in the form of a situation where you get $2.50.
Especially if you keep some money for yourself, it's a good deal.
It seems to me that either it it's better for SIAI to have the money, in which case it's a worse deal, or it's better for you to have it, in which case keeping all of it is the best deal. I don't see why keeping some of it would be optimal.
If I gave you $50 you hadn't planned on receiving, would you consider giving it to charity?
Here's your chance to find out.
Just in time for the Tallin-Evans matching fundraiser, ING Direct has started offering a free $50 cash sign-up bonus. I've personally used ING for 10 years and referred over 20 people to similar promotions of theirs in the past so I can confirm that this is legit.1
It's a simple, effective way to get started as an optimal philanthropist for free:
Full disclosure: I was an SIAI Visiting Fellow in 2010. I've also used ING Direct as a customer the past 10 years, but otherwise have no financial interest in them.
[1] This isn't one of those bogus "intro" deals where you have to make sure you cancel the service later on or risk getting charged fees. ING has no fees, no minimum balance requirements, no sleazy marketing emails, and consistently good savings rates. If you want to use them for their good service after signing up, great, if not, no worries. All you have to do to qualify for the $50 is make 3 small purchases you were planning to make anyway with their new debit card. I know it's a trivial inconvenience, but I think it's worth it to be able to donate up to $100 to charity without actually spending any money.
[2] Profit denominated in warm fuzzies, karma, and post-Singularity catgirls.