There's an issue without you saying not because you don't know econ 101, but because you do know it. Because you shift the focus from the systems (global imperialism) to the individuals ("so you shouldn't be allowed to migrate?"). What causes migration is, objectively, unequal development of different countries caused by imperialism and inherent to the market system, and not "personal decisions". That means shifting the talk to "personal decisions" is pointless and harmful.
It's like going "oh but you voluntarily choose to buy/sell" and blaming all your economic problems on yourself.