Of course, Elly's assumption that empathy means that people think as she does isn't the only example of brain failure that makes her do stupid stuff like this, After all, she and her fellow mommy-jackers not only see themselves as rescuing their children from the delusion that they get to decide what they can keep, they also see what she's doing as sacrificing for the greater good. Said greater good means helping him by getting rid of things that get in his way and waste his time so that he can concentrate on What's Really Important.
As it so happens, the Really Important Thing that Michael has to devote his life to is to allow Elly bask in his reflected glory. In the strips that are being re-run as I type this, he HAS to become the greatest trumpet player who ever lived or ever will live so that people will gaze in awe at how great a mother he must have. As time went on, Elly got to live through him when he became the Saviour of Canadian Literature. What Elly lost sight of is that deciding that Mike had to live her dreams because she could not, she'd essentially said "Well, my life is over. The only way I can ever achieve my dreams is to make my children do what I'm too old to do."
Sadly, since Mike yearned pathetically for Elly's approval, the man he is right now probably agrees that Mom was right to just swoop down at random and decide that only she got to decide what he can own. After all, he was too weak to defend it so he didn't really deserve to keep it. That being said, it seems likely that his thinking that he simply had to remind Lizzie that since she was weaker than he was, she had no real right to anything like affection is probably another thing that idiot Elly thinks that television taught him. It's like watching her not realize that she taught Liz that other people's belongings have no value whatsoever so asking her to worry about them is just wrong.