The character designs from Lego are indeed a special case. They are not in the same category as fan-created designs, because they are what defined Bionicle in the first place and inspired the fan-created content. Yes, it does matter that Lego, not a fan, created them, because Lego owns and created the IP, and that inherently carries more authority.
On the other hand, I am saying that some arguments in favor of running this contest, like the one you are offering, invalidate their own premise. Namely, they are claiming that canon is significant and at the same time has no significance. I prefer not to run the contests because I think there is too great a risk that I will not be satisfied with the results, and I am content with fanon, some aspects of which I enjoy, and some aspects of which I do not. I do not see a need to impose one canon design on the community, and I think it is better to leave things to personal interpretation. Your rebuttal is that the canon contests do not need to impose on other fans' interpretations. But that implies that they have no significance in the first place, and so are not canon. That is what canon means. It is an absolute, agreed-upon interpretation. My issue is not that it would prevent anyone from making MOCs -- of course it wouldn't. My issue is that I would simply not like the canon design, which does hold weight with me, even if I have the ability to have headcanon.
So you are saying that the canon models have significance even though people are always free to have headcanon, in which case I vote no.
But more fundamentally, there are two mutually exclusive possibilities, both of which lead to the same conclusion for me. Either canon has meaning, in which case I don't want new canon MOCs that I don't like, and I vote no on this issue, or canon has no meaning, and the contests have no value, in which case I also vote no. That is my logical reasoning.
Then you are going to have to justify to me what we have to gain by canonizing designs that we cannot also gain by developing fanon.
@Mesonak My opinion, FWIW:
Not a fan, but if it is limited as you suggest, it's not a big problem for me.
Really not a fan, because they are not compatible with the original Bionicle head-mask system.
What about custom armor pieces?
This seems okay. Would the art contest have guildelines consistent with the Lego theme, or would movie-style designs be encouraged? I think submissions for the art contest should be required to have enough angles/detail to build an accurate 3D model at the end.
I'm still in favor of either having a separate step to decide what to hold a contest on, or a separate step to decide whether the contest winner is canon, in order to alleviate contest bias.