I think the point is that you have to learn how to train and that includes the role of uke. It helps if there are clear levels of resistance and even clearer lines where they are to be used.
1. It is a trivial manner to shut down a technique you know is coming. If we always did that the technique would never get learnt.
2. If there is absolutely no resistance tori will never feel how the technique really works and the technique would never get learnt.
The role of uke has to be taught as much as the role of tori.