Encryption in cellular systems is to protect over-the-air signals. It's irrelevant when it comes to 99% of legal interception because for that law enforcement simply asks the network operator to share the plaintext traffic from within the network.

If you want that no-one be able to evesdrop then yes you have to have your own encryption on top. These days a lot of data already goes through TLS but for instance standard voice calls are obviously transparent to operators.