Is it a crime to write a book about one's experience working for the President? The book, in fact, was highly complementary to President Bush (have you read it? -- it's actually quite good), yet it was also honest, I thought -- but overall positive. He's a writer. One can't fault him for writing a book -- it's what he does.
I admit that I haven't read Frum's comments on Hagel, so I can't comment on whether he did or didn't impune the Senator's patriotism. But the fact Frum's from Canada doesn't disqualify him from expressing his opinion. In fact, one could look at it this way: Frum chose to come the US; Hagel enjoys being a US citizen by the "accident" of birth. Frum made a conscious decision to live and work here. That qualifies him to enjoy the freedoms to express his opinion, even if that opinion expresses opposition to a U.S. Senator's views.