Not sure if your being serious, or not. In the event you are, keep reading.
That's like saying if terrorists are allowed to exist, this will only help innocent people die.
Whats to stop someone from using a third party app to encrypt their data? WhatsApp and others. It also wouldn't be that difficult to develop a groups own encryption software by using readily available libraries. If strong, backdoor-less encryption becomes illegal in the US, then grab software from another country.
Also, a phone thats easy to get into by the gov't won't be that easy to get into when its destroyed. Yanno, like the personal phones of the terrorists in question.