I don't think the president has or should have the power to change the law which describes what is a machine gun. It's definitely a power grab. People were right to criticize Obama for overreaching his authority with EO's. A pen and a phone is not sufficient to overcome designed checks and balances. I think ordering the government to do studies and stuff seems within the boundaries of presidential authority.
I'm not sure if this is really a defense of Trump, but I don't think he understands the structural problem with his bumpstock memo. I think he thinks that, as POTUS, he can just tell an agency what he wants, then let them decide what to do about it.
And before anyone jumps my **** about thinking Trump is dumb, I don't. He's smart guy. But this is pretty deep governing-stuff. He doesn't know what he doesn't know. And, I don't think the people around him are particularly enthused about explaining to him why he can't do things certain ways that he's used to doing them.