Where do you think all that plastic is? The only plastic I've ever seen in the bit of a braking system that one would use brake cleaner on, the calliper and disc, is the rubber in the hoses and seals (generally EPDM rubber because they have to resist hydraulic oil).
Most pads are plastic backed to eliminate the use of grease, then you have the wear sensors, guide pin sleeves, anti squeal shims etc. There's then everything in the surrounding areas which will get sprayed during cleaning, arch liners, abs sensors, just about every other sensor...
Brake cleaner gets used for everything in an automotive workshop though, particularly cleaning oil and grease residue in the engine bay.