VS arguments

I have a question about Vs debating.

When people debate manga they usually go by the original author as everything else is considered non-canon. If so,how come it is okay to pick a character for let's say comics that has several writers but call all of them canon because the company says so?

When people talk about versions in comics,like Pre-Crisis Darkseid or New 52 Batman. Shouldn't versions be determined by the different writers or the different series as opposed to the different eras? should one debate for instance Grant Morrisons Batman vs Archie Moore Joker or perhaps New 52 Action Comics Superman vs Superior Spider-Man?

Why are Vs matches set up by eras as opposed to comic series or writers?

@lifeshaver: This is because often past comic series are referenced in newer ones. Also the fact that it's one large continuity. The writers write to make up this continuity therefore it's all one large canon.