Arrests happen all of the time, and since most people in America actually have cell phones, than when they are arrested, what could be considered to be for many people their most important and personal item that they own, is now essentially in the hands of the police. Of course, there are laws, laws of seizure, laws in regards to a warrant, all sorts of laws, and permissions granted or implied, which can protect citizens from unlawful search and seizures, in which the Supreme Court has made it clear that cell phones cannot be searched without a proper warrant, but cell phones are still searched, anyway.
The police are very good about applying pressure on suspects, and when the police have an arrestee in an either-or situation, in which, the either is to give up your cell phone voluntarily and the or Is to be arrested and taken to jail, than you as individual are really up against the wall, for once your cell phone leaves your hand, it doesn't really matter how "secure", or how "encrypted" your phone is, because unless you, yourself are some sort of professional hacker encryption expert, the police typically have all the tools that they need to "unsecure" your phone, or to "un-encrypt" your phone, and hence everything that has been done on that cell phone of yours, is available for their perusal. This means that the voluntary relinquishing of your phone so as to not be arrested, most probably gives the police a portal to the most intimate aspects of your life, as well as giving up the connections that you have to people that are part of your social network, which may, of course, endanger them as well. Not only that, police like to make promises and not honor them, so that, even when there is supposed to be a quid pro quo, they still arrest you anyway, so all things being equal, you are probably better served by insisting that the police get a warrant to search your phone.
The fact that cell phones are not legally searchable by the police unless under warrant or unless you give explicit permission that they can do so, doesn't mean that they won't be searched anyway, for once the cell phone is out of your hands, you aren't going to really know for a certainty what happened, and even if you are able to put together forensics clearly showing that your cell phone was searched, without a warrant, proving so in an actual court of law, is problematic, at best. The police know all this and quite frankly the police don't really care, for at worse, there isn't really any penalty for them, except perhaps, the suppression of evidence in that particular case, but why would the police worry, because now they have all of your personal information as well as your contacts so they can easily arrest you or one of your contacts at their convenience on something else, or simply put you in their vast database to monitor and to track.
Still, it gets even easier though, for the smartest police don't want to take the chance that their evidence will get suppressed, so for certain suspects, they will play it by the book, and actually get a warrant to search your cell phone, and those warrants are issued by certain judges routinely, indicating, that the convenience and ubiquity of cell phones that carry a significant portion of our personal thoughts and activities are readily available for the police and that should definitely deeply disturb you.