I think OP was suggesting that it's the responsibility of those companies to warn their users, etc. Not that it was the firesheep dev's responsibility.
exactly, it's MITM attack people have been able to do for at least a decade
That doesn't really make it less of a security threat. Sites still don't know how to secure it. On my own site we do the same, and have no other answer but end-to-end encryption which we then charge for.