Why do you say he is well-versed in copyright and patent law? He has always been vocally against patents and even threatened to leave id Software back in the day if they started patenting. He is a known proponent for open source and have released a lot of source code for their games. I do not think he is well-versed in this at all, because he basically wants nothing to do with it.

Because he ran a bloody rocket company and a software company and that means that you have to understand the basics.

Or did you feel that 'being a nice guy' should protect you from legalities or the responsibilities that go with the job title?

Know your enemy... Just because he is an open source proponent and against software patents does not mean he isn't well versed.

Nor that an aqui-hire of a succesful studio like id software would go down unlawyered.

Wow. I missed the "unlawyered" part. If so, that was dumb.

Because you often expect people to actually be reasonable... despite the fine print.

Also, because he's human, and they all unfortunately make dumb mistakes.