Are cops allowed to go through your phone?

[It's Nanerpus!]

Apparently I was "trespassing" by sitting on a wall watching the trains go by (something I do frequently) and these cops arrested me. While I was in cuffs, the one cop took my phone and started looking though it. I asked him to turn it off and he said he would, but he still kept looking through it right in front of my face and didn't stop.

Are they allowed to do that? I don't see why or how it's relevant to what was happening.