Subpoenas are based on information you have. Where is the law or regulation that says they need to keep it? Telcos keep it because it helps them with network

Subpoenas are based on information you have. Where is the law or regulation that says they need to keep it? Telcos keep it because it helps them with network capacity planning and is incredibility financially lucrative when they want to sell the data. It's probably more to fill in their data product for malls and fine grained location than to do it for subpoenas, which if they had a choice would probably rather not have to do.