They still need to charge you with something. If they can't, they can't hold you.
Yes, I know, they can theoretically do whatever they want, but realistically it would take the most spiteful of spiteful agents to arrest you without cause (just because you refused to unlock your device). Just the act of doing this would create a lot of extra work and paperwork for the agent that most of them are not going to want to deal with. Plus, asserting your rights is a sign to them that you aren't a pushover and aren't going to get trampled on easily. The bullies prefer easier targets, usually.
Yes, I know, they can theoretically do whatever they want, but realistically it would take the most spiteful of spiteful agents to arrest you without cause (just because you refused to unlock your device). Just the act of doing this would create a lot of extra work and paperwork for the agent that most of them are not going to want to deal with. Plus, asserting your rights is a sign to them that you aren't a pushover and aren't going to get trampled on easily. The bullies prefer easier targets, usually.