The main reason is probably to lock you into their store for ringtones and games. Their guise for it all would probably be so-called security. At least that's the impression I got while I was their customer.
Check this out, to actually DOWNLOAD apps from their store they made you pay some sort of extra charge. Paying them money for apps wasn't simple enough. I passed and got busy modifying the firmware in a hex editor.
If you visit a Cricket location you'll see mostly poor folks who can't pass a credit check, and Cricket milks them hard for basic features -- like caller ID. To make matters worse, their coverage rarely goes beyond the city; travel 25 miles out and bam -- no service, no roaming even. It's an all around bad deal. For me I wanted a no contract plan since I hoped to move overseas.
Trying to cancel my account was a pain. The customer service people on the phone kept offering me new deals of some sort -- totally ignoring that I've moved to an area without coverage, and I reiterated that fact repeatedly.