[avar]

Since Redis is BSD licensed I wouldn't be surprised if this is the start of popular paid-for proprietary third party modules.

[byroot]

Even if redis was GPL licensed you still could do that. Linux is GPL and there is proprietary driver modules.

[ploxiln]

There are proprietary modules for linux, and they are distributed and used, but there isn't really a full consensus on whether they are legal. You can certainly do it and get away with it though.

http://yarchive.net/comp/linux/gpl_modules.html