Employers are subject to substantial penalties if they hire someone who is not legally permitted to work. I think they are woried about more than unexpected delays, though I admit I'm way out of my sphere of knowledge on this subject.
To be clear, I meant "unexpected delays (to work legally)". As in, waiting a year to submit, and hope for, an H1B to be issued, vs waiting 2 weeks for a TN to be approved.
No portion of my initial comment should have been interpreted to mean that employers do not do their own due diligence after actually hiring someone.
