Yes, you are far too skeptical. Windows is not really the issue you think it is, except among the Windows haters.
Only if you compare it to nothing controlling the machine. The machine has to have a control, which is going to cost money.It looks like WinCNC also adds about $1000.00 (possibly more) to the price of any given machine.
I think that Camaster and Shop Sabre have decided that WinCNC is the most robust "low cost" controller available.
The alternatives are the lower cost "hobby level" controls, which would probably be a support nightmare for those companies, or higher end industrial controls that can cost a lot more than $1000. Unless you were to go for a chinese control, with no or limited support.