I think because alot of fans had it set in their minds what SM3 was going to be until rumors came out that Maguire and Garfield would make an appearance ,and that it would be a multiverse film . I think the idea that the film could be going in a direction that they didn't count on has thrown alot of fans for a loop, because it may not play out the way they've predetermined it to play out.
I also tend think it's probably generational in that sense that, you have alot of younger fans for whom the idea of Maguire and Garfield really doesn't resonate, while its an exciting idea for older fans who grew up with them as their Spiderman and to the General Audience as a whole who consider Tobey's Spiderman iconic.
So, I tend to think the "we don't want Garfield and Maguire" is the majority opinion among a younger cohort of fanboys, but the minority opinion overall with fandom in general ,and certainly the minority opinion in comparison to the general audience. That is to say, The GA wouldn't be against Maguire and Garfield coming back as the younger fans seem to be.
Now, the "we don't want Tobey and Andrew" view is a different issue from believing whether or not they're in the film. Then again, I suppose if you don't want them to be in the film ,you may be more likely to believe or want to believe they're not in the film, and visa- versa.
