What does this even mean? He either pays it or he doesn't. "Within reason?" Five dozen people injured. That's millions of dollars. He should pay all of it? There is no "within reason" here. A person probably earns 1-2 million in their life. So would within reason mean he pays like $100k? He doesn't have that. Sorry to go grammar police on you but you can't just throw out cliches and sayings just because you heard it somewhere. In this case, it doesn't make any sense.
As long as they're within reason, he should pay them all (e.g. not half). While I do see your point, I believe the statement (and its conditions) can be reconciled (grammatically/logically).
Within reason could mean a lot of things. (Unreasonably high direct costs of (perhaps experimental) treatment (including long-term), or unreasonably high indirect damages (let's say he murdered a top scientist on the verge of discovering the cure for cancer or whatever). You get the picture. It's just a way of hedging against the unforeseen.)
If you'd stop second guessing the meaning of his post, and simply took it with a grain of salt, you'd probably go easy on him as he's just pulling the OP's leg.
Correct, however since I'm not a native speaker, I do have to take constructive linguistic criticism into consideration. (I guess native speakers should too, but you get the point.)