It has to do with nucleation points and the resulting effects I think. You can certainly carb things other than water with a soda stream, it's just that it has to be really cold, and you might get foaming/spraying, and that the nozzle may become clogged by residue (sugar) and that using it invalidates the warranty, etc etc etc.
Carbing beer with sodastream, or even a carbonator cap, is usually only done for a brief test during bottling, or prior to kegging to see what it tastes like (and even for kegging, the carbonation period can be so brief that its not worth doing).