So all your data go to Apple instead. And many apps collect data and send to Google (eg. Any Google app, like maps or waze).
There's very little reason any more that a phone can't be used in the US, it just needs to support US frequencies. Cell providers like US Mobile don't care what phone you bring, so long as it's compatible with US frequencies.
Plus I'm sure someone building an OSS device isn't going to ignore an entire market of frequencies.