That has more to do with the PS3 than the Blu-ray format, though. There's stuff going on inside the Cell which makes it more or less impossible to run unsigned software on it (except for running stuff through Linux, of course):
"In addition, there is another security layer called the Hypervisor running on the PS3. The Hypervisor can be considered the lowest layer of the operating system and runs on the PPE and the one reserved SPE with the highest priviledge. Encrypted code can be secured by having it run on the one reserved SPE, which uses hardware root key and hardware decryption routine for decryption. The PPE would take the encrypted code (can be analogous to an application file from the decrypted Blu-ray disc, or something from the flash memory) and setup a SPE to go into secured (isolation) mode. In this mode, the hardware decryption routine takes over, grabs the encrypted code, decrypts it using a hardware root key, and puts the decrypted code inside the SPE's local store."
http://www.edepot.com/playstation3.html#PS3_Security
(Great site, with a lot of nice info about the inner workings of the PS3.)
Of course the data on the BD is encrypted as well, but what I'm saying is that the BD format itself isn't the reason why PS3 piracy is at 0%.