They are responsible for the design of MCAS.
are they? I doubt R-C have aerodynamics experts on the project, as such they would have little say in defining control laws. I don't believe Boeing will be sharing intimate information about plane behavior, more likely just provide specs with "need to know" attached. and if those specs were followed, Boeing cannot get much more than "too bad, so sad.."
So if Toyota had a dangerous cruise control issue with a module supplied by another vendor, you think Toyota should be the only one blamed?
I suppose you thought Parker-Hannifin was blameless as well.
Project is divided between multiple companies, departments and people, each of them having their role.
Someone defines what FCC should do, someone converts that into laws and algorithms, someone codes them into C(or whatever they use), someone else tests resulting software. There is only that much overlap between different stages, and a coder can do only that much if aerodynamic engineer wrote "+" instead of "-".
I don't know who should be doing things like defining erroneous input response definition. I suspect that is still done on Boeing side of things.
That is why it is very important for the head company to define specs carefully, without assuming something is obvious to people down the line. Besides, there should be (and most likely there is) a contract defining roles of R-C and B in this case.
It is entirely possible R-C is totally blameless, and it is equally possible they carry a good share of responsibility. However, there was an assumption that pilots must recognize and correct certain conditions, and that assumption was certainly made by Boeing. It is not up to software folks to question those assumptions.