Illegal?

So, lots of authors have FAQs on their websites.  And a question that often appears is:  Can you read my work?

And the answer is always:  No. 

Most often, the explanation given is that the author is terribly busy with their own writing.  Sometimes they also say that is "illegal for them to read unpublished work."

Um.  Is this true?   I'd never fully paid attention before because, you know, it didn't apply to me.

But now, I'm wondering:  Is there going to be something in my contract when it arrives that says I can't read unpublished manuscripts?  Because, yeah, when I'm busy with my own revisions and/or new projects, I probably won't have time to do critiques very often.  But if I do want to, will it really be illegal?  Or is that just a handy excuse?