I understand that software directly and silently changing the default software can be a security issue. But it's only because it happens silently. Does Windows allow for showing a system prompt that confirms the change to the user? If not, then that's just plain ol' anti-competitive. Especially with how pushy Edge and Bing can be.
I don't think the goal is to lock you into their browser, since you still can change it through the GUI. It seems to be part of the recent push to block software which changes hidden settings. The end goal being to lock down the OS and prevent users from disabling features MS wants to push onto them.