Because:
There’s no way to sync contacts and calendars between an iPhone (and other mail clients) and protonmail. The app does one way sync from the phone to protonmail, but not the other way round.
8 years ago a feature request was made to add support for CardDAV and CalDAV, but even with the release of bridge it’s not there.
So iOS users have to resort to using other calendar services, or 3rd party bridges to enable it.
But none of that is going to stop them from detaining you until you give them the pin.
US citizens might have it a little easier. But foreigners are certainly going to regret their choices if anyone ‘close’ to the border has an issue with them.