My phone that I purchased is not "their" platform. Better analogy would be if Wal-Mart sold me a fridge and then somehow managed to make it so I can only store groceries purchased from Wal-Mart in that fridge. Now if anyone wants to sell me groceries they need to sell them to me through Walmart, otherwise I can't refrigerate them.
As long as you understood the limitations of the fridge you purchased, i.e. you weren't defrauded, what's the problem? Do you really need a nanny state to prevent you from making bad purchases??