Moto cannot prevent leaks, because even if they totally changed the authentication method for their dev servers, an enterprising developer would simply rewrite the cheesecake app to work with the new method.
Moto doesn't really want us running their pre-approval OTAs, and as a matter of fact they don't even allow discussion on their forums. If they were truly allowing leaks on purpose to get free testing, they would encourage discussion.
And in Moto's eyes, none of us are even qualified to beta test their software. To be honest, we're not, they have a very specific checklist they go through. Our testing is completely random, and useless because many of us (i'd venture to say MOST of us) run 3rd party apps. Which totally invalidates a true beta test, since 3rd part apps can have far reaching effects on the OS stability, and it's not Moto's responsibility to track down incompatibilities with apps that aren't on the phone stock.
EDIT: If Moto really wanted to provide a consumer beta, they simply would allow everyone access to the OS, much like Microsoft is doing with Windows 8 for the PC.