Why not?
Personally, I don't think I would turn down free money regardless of how much I had. Especially after a lifetime of cultivating this idea in my brain that more money = more freedom, it's hard not to want more. And as petty as it is, I do like seeing big numbers when I log in to my accounts :)
On top of that, the 4% rule as stated by MMM does not guarantee 100% success. So even if his confidence in the 4% rule is exactly as he claims, more money still increase his chance of success.
As for the charity question, who knows? he may being giving lots to charity, he may plan to give it all in his will, he may have plans to start his own charitable organization, or he may do any other thing you can possibly think of with it. You seem to have a very narrow view of what might be going on in the life and mind of someone you've never met.
I agree, even if I am FIRE'd in the truest most conservative ridiculous definition (2% WR on Fat Fat Fat FIRE with pensions and HC covered) and somebody wants to pay me for something I am already doing for fun/hobby/whatever....I am definitely not saying no.
And the 4% rule is not guaranteed, so why not for that reason too. And shit happens (like divorce) that can f things up, so why not take the money for that reason too.
As for the charity, I think I recall that he said he would put most of the money toward charitable pursuits but I think he has only evidenced about $100k. And because of the more recent twitter comment that said he would be ramping up the blog to buy a house suggest that minimally his intended purpose has changed.....not too mention some or a lot of the money has also going to building up his assets in other ways too. So the $100k was nice, but the indicated saving of the world was not completely followed through.
And for the definition of FIRE - for me it falls under the following:
A) if your investments can support your "DESIRED" lifestyle then you are FIRE'd
B) it they can't, then you are not.
C) possible exception is one time expense...like maybe you want to take your family on a bitching month long safari trip to Africa that all in is $20k so you decide to go to work to cover the expense (this only applies if its a one time desire and not recurring expectation of this kind of travel every year. But if you budget $5k for travel and decide to forgo other travel for the next 3 years - then still FIRE'd.
* I think it also possible to move between the three scenarios as your Desired Lifestyle evolves that can result in inflation or deflation - after all peoples interests/desires can change.