Who says so?
I'm not picking sides, but I try to help myself understand things by visiting both sides of the arguments and there is clearly a side interested in cutting down on fraud, just as there is a side promoting freedom in their computing. Limiting sideloading seems a sort of pragmatic move, and leaving the possibility of sideloading open seems to be likewise a pragmatic move, especially if there are guardrails and warning signs everywhere, we do this in society when faced with hazardous situations.
I am very open to counterpoints here, at least if I can read them before they get too greyed out!