You’re not, though. You’re accepting to the company. The hiring manager may well quit or you may be reorg’ed at a moment’s notice to a new manager. I’ve been a hiring manager and I would be bummed if someone accepted and then took another offer, but this is their livelihood and their life we’re talking about. You do not owe nor are you owed fealty to the hiring manager or a company that may very well kick you to the curb at any time.