I've paid for many instructions, big (1:8 cars) and small (e.g. 42093 Z-models).
I don't see any problem about it, I understand it takes a lot of work making instructions. I have even paid 20+€ for photo-stream instructions and I'm happy about it.
The only points I could complain about are:
the professionalization of some people who seem to be putting out new models quickly to make money. But frankly I realize that's my own problem of being frustrated having too little time and resources to build everything.
paid instructions that "should" be free because the result is so small I can build it from the picture.
In the end people do whatever they want.