Your argument implies some type of biological monoculture, i.e., all crops will behave the same as canola. On what basis do you make this assertion?
Also, your argument would be a bit stronger if canola wasn't the crop referred to in the study the BBC post is talking about.
It's a known issue that colonies cannot survive long term on substitutes. Why this is isn't clearly known yet but the assumption is that they are missing some dietary analogue to vitamin C.
In fact: the idea that bee colonies would be stressed by a diet solely of oil rape nectar (note: no commercial bee colonies have such a diet, because they're moved around to take advantage of growing seasons elsewhere) favors my argument: it's another way in which bees working canola crops should do far worse, given neonic exposure, than they actually do.