Why does "拒不" mean "to refuse to do something"?
It seems that there are some teachers in school think the same as you do.

This also shows that there may be still debate in Chinese academia about how to explain this, so there might be not a definite answer.

Also, 拒不 is still not considered a word. But as it is used more and more frequently, it is becoming one.

Another article on CNKI might shed some light on how to understand it, (but I cannot access the full article, only the abstract) which lists some similar phrases:

拒不VV
拒而不V
拒绝不VV  

(a little hard to decipher the symbols in the abstract without the full article) The author seems to be claiming that it is two negative sentences "mixed" together.