It isn’t illegal to side load another OS with another App Store onto your phone. But Apple is also well within their rights to design their phones to brick if they detect something they deem a security risk being loaded onto a device they sold. They tell you up front, before you buy it, what the device is designed for and what it isn’t. Xbox doesn’t let you run Word and won’t support it if you somehow figure out a way to. Is that not the same thing?