Hmmmmm....

AND, from a marine:

If the question is "Did the Marines who greeted the President willfully show disrespect by failing to execute a "facing" movement after the salute?" the answer is no.
Proper protocol is for the Marine to snap a salute to the President. The President may or may not return the salute at his discretion. If the President returns the salute, the Marine immediately "cuts" or "posts" by bringing his arm sharply down to his side from the salute and remaining at a position of attention. If the President does not return the salute, the Marine will wait until the President passes him, then he will cut and remain at a position of attention. This is the protocol when greeting a senior officer, including the President.

With the new Presidency comes a changing of the Presidential Detail. The new command of the Presidential Detail may have chosen to add a facing movement to the protocol. This is at the complete discretion of the command. A Marine does not have the luxury of choosing whether or not he follows proper protocol. He is given an order and he follows it to the letter. If the order does not include a facing movement, the facing movement will not occur. If the order includes a facing movent, the facing movement occurs. It is that simple. Whether or not this movement becomes a standard part of protocol remains to be seen.

Marines do not act as individuals, we act as a team. Marines follow procedure and protocol. The Marines you saw followed their orders. That is a Marine.