I know I'm late to this party, but so much ♥ ♥ ♥ ♥ for the assessment, it does seem like that's the source of post-DH Snapewank in a nutshell.
I'm more baffled by how anyone was honestly expecting that in a series titled "Harry Potter" when Snape didn't even make the Main Character list. Oh he's absolutely one of its more important characters, the only ones more central to the plot's drive are the Trio, Dumbledore and Voldemort. But seriously, the guy's never appeared in more than thirty pages of any given book.
I don't know, but I think it's the same as what's behind the Hermione-fanatics. Harry's function as the main character so far as these people were concerned was to correctly validate and adore the character they personally identified with. By not receiving that validation from Harry (and thus, from JKR), they have been cheated and wronged.