There's one essential difference between him and professors, noted subtly in the "it's part of their job, remember?" - he doesn't receive a teaching salary. Whether receiving public money should automatically mean that your writings should be freely available for all of the public is another discussion - the point is you can't really conclude he's a hypocrite.
The other point that's being made is that textbooks mostly consist of old knowledge, established long ago by other scholars. Godin sees a difference between selling new ideas and recycling old ideas. I'm not sure I completely agree with this train of thought - sometimes translating complicated academical concepts to make them accessible for students is quite an effort in its own right, and thus worth money - but I don't completely disagree either.
The other point that's being made is that textbooks mostly consist of old knowledge, established long ago by other scholars. Godin sees a difference between selling new ideas and recycling old ideas. I'm not sure I completely agree with this train of thought - sometimes translating complicated academical concepts to make them accessible for students is quite an effort in its own right, and thus worth money - but I don't completely disagree either.