It surprised me that Marrisa Mayer was allowed to quit Google and then start at Yahoo the very next day, is that common?
Its CA law. From the business and professions code [1].
[1] http://www.leginfo.ca.gov/cgi-bin/displaycode?section=bpc...
In other words, nobody can enforce a clause preventing a programmer from programming for company B after leaving company A purely on the basis of restricting competition.
They may have a case if e.g. company B contracted company A to do some work and then bought out company A's programmer (e.g. to save money), but only if the contract between company A and the programmer in question has a restraint specifically penalizing this scenario.
(Even then, it doesn't mean companies will choose to enforce the contract. I've worked for companies where the restraint was between the companies A and B, and in the interest of preserving business relationships, company A decided not to pursue their legal right to recourse when company B poached an employee from A; the employee did not break conditions of their employment with A as the restraint was between their companies.)
"It also only applies if one is fired from said previous employer, not one who's left at will."
?
Are you referring to California here, or non-competes in general? If the latter, I'm, pretty sure that you're incorrect. " The use of such clauses is premised on the possibility that upon their termination or resignation, an employee might begin working for a competitor or starting a business,..." (top of same Wiki page)
Also, fired/quit doesn't affect non-competes.