Every time I bring this up with an American teacher they go into this long litany of how busy they are and how poorly they're paid and so on. I don't give a fuck. If you are using an answer key, you are No Good and I don't take you seriously. I refuse to give a fuck about any of your other problems if your brain is not minimally engaged with the subject material in which you purport to be competent.
It's common sense to run through a homework assignment to certify the key they're using. But I don't understand how it would be logical to suggest they're no good as teachers unless they spend another 1..n hours a night writing all their assignments by hand and not using prepared material from the books.
I'm well aware of all the pressure on teachers to act as substitute parents, social workers and so on. But none of their input on these subjects has any value when its divorced from a foundation of academic rigor as their first priority. Without that foundation you cannot have any political leverage. American teachers seem to have forgotten this basic political truth. If the general public does not perceive teachers are intellectually competent and engaged, then it is not going to give any weight to those teachers' opinions on other topics. A degree is education is not a substitute for command of the course material, it's a complement to it. If you can't do a problem like 3 x 7 x 26 in your head then you are innumerate and should not be allowed to teach a math class.