I'm not claiming it's illegal. I'm just arguing that this is a new and serious security concern. You write:
> This technology isn’t being used to break into phones at surprise checkpoints, it’s being used to search phones of people who have been arrested.
and:
> As far as I know, the government isn’t stealing peoples iPhones to search them.
That implies it's nothing to worry about if you aren't being arrested, which is wrong.
First, you don't know when this technology is being used. It would be prudent to assume they US government could use this technology on any phone they seize.
Second, even if it's not "stealing" when government agents seize your phone at a border (or yes, at a surprise checkpoint, which they can and do use), from a security standpoint, it's the same thing.
The legality of these searches is not that interesting to me (witch-burning and slavery were legal too). What's interesting is that this new exploit, assuming the story is accurate, allows the government to search the data of phones that they seize.
Why should we worry about that? Because, as we have already established, they seize phones routinely, and not necessarily in conjunction with an arrest or even suspicion of criminality.
Yes, it's legal (in many cases, anyway). But before this new phone-cracking capability, it probably wasn't effective. The security on the Apple iPhone was believed to be good enough to stop such intrusion; now (again, assuming this article is accurate) we know it isn't.