All one needs to do is enter "Charlie Hebdo" in Google Image Search. It is offensive stuff, and pushes beyond the limits (at least my limits) of parody/common decency. I would be ashamed to publish it or have my name on the masthead. I am against violence. I am for freedom of expression. It does not follow that I am for Charlie Hebdo.
To try to repair your analogy, what if the Westboro Baptist Church were to demonstrate a lot of redeeming qualities like actively opposing right-wing reactionaries, crazy leftist social graft, and (of course) radical religions? What if they were to spend most of their time exposing issues that seriously deserve ridicule, and God Hates F@s was an occasional anomaly, a result of pushing the limits too far? (I never read CH so I don't know how anomalous it is. I do know this though: google image search can be pure selection bias and is definitely not a good way to get a feel for the magazine as a whole)
I personally dislike like Charlie Hebdo the same way I dislike like Benny Hill. The occasional mild laugh just isn't worth the tedium and distaste. (and this goes for other BDs like Fluide Glacial, L'Écho, the whole style)
So, I'm not for Charlie Hebdo either, but I'm very happy to see the outpouring of support. It's clear what people mean when they say je suis charlie -- no need to parse and overanalyze it.
It's a false dichotomy set up by people with an agenda.
Who exactly are you talking about?
You might be mischaracterizing what some/many/most people mean when they say, "we are charlie."
We must condemn the murderers but we don't need to endorse or ally ourselves with the publication. Most people who repeat the "We are charlie" slogan are probably not aware of the publication's racist history.