The basic difference is that Amex MIGHT give you a Financial Review, at least in theory giving you a chance to defend yourself, whereas other issuers will just shut you down. I'm not sure that Amex is really more tolerant of any given quantity of MS (or quantity relative to CL or something)

And it also varies greatly over time. In April/May 2013, Citi was aggressively shutting down all Citi cards for those it decided were abusing the Citi 5x TYP. Once this stopped (and there was no obvious pattern as to who got hit, both light and heavy users) Citi seemed to allow unlimited MS on the card.