That's really a great question. I think, if it's true that DM is guilty of 1st and MS isn't, in DM's mind the way he knew MS would just go along with it is the same way he knew MS would go along with all the other crimes they committed. Their relationship dynamic seemed to be one of supply and demand. DM had the supply and MS had the demand.
DM kept MS around by all the goodies he had to offer and MS stuck around for those goodies. I suspect if you took away the money, adrenaline rushes the thefts provided, the parties, the properties, etc., MS likely wouldn't be all that interested in DM's company. But, that's just a guess. I could be totally wrong.