They likely let him go as a favor to Schwartz so he's free to take any job offered without getting the Bill's permission. It was difficult to imagine that a former HC who had the #2 or #4 defense (depending on who you ask) was going to want server under a head coach with a different philosophy. In order for Schwartz to get back to being a head coach, he'd need to go elsewhere. Also, the Bills passed him up in favor of Ryan.