Console manufacturers sell consoles with a loss. Therefore, they need to have a monopoly on the software. However, homebrew is a way of software development that doesn't hurt their position, and there is demand for it, not providing a homebrew option increases the interrest people have to crack the protection. Therefore they provide a "crippled" environment where people can experiment as they wish.

For XBox-360, this is XNA, for the Playstation this is a sandboxed Linux. Now, I'm not sure what Sony wants to achieve. Of course if you have full power under Linux, third parties can release software for the PS3 through the Linux option. But this hasn't happened yet, so there should be no reason to block access to the gpu...