Yeah, PG-13 cartoons tend to go further than PG-13 live-action projects. In fact, a lot of them (Including Hulk Vs. Wolverine) would no doubt get R ratings if they weren't animated. I think BECAUSE of the stigma about cartoons being for kids that you mentioned the producers of these cartoons go overboard in trying to be "adult".As Fone said, a good portion of PG-13 cartoons go further than a live action PG-13 film just for the heck of it.
Much like Marvel and DC Comics, cartoons have always and still do carry the stigmatism of being kids stuff. As a result, many adult cartoons overload on adult content just to prove their so called maturity.
In the case of this film, it's necessary as a way of showing how lethal Wolverine can be, but many animators who get the freedom to do a project for the older viewers just take the kind of plot you'd see in a kids show and fill it up with adult content (MTV Spider-Man episodes except the finale are a good example of this) rather than a series with genuine maturity (DCAU, Gargoyles and The Spectacular Spider-Man).
Anyway, if anyone has something else to say on the matter, PM me to discuss your opinion.
Frankly I'm a little annoyed Hulk Vs Wolverine and Batman: Gotham Knight didn't get R ratings as they are COMPLETELY innappropriate for kids and about ten times more violent than PG-13 live-action projects that parents DO find acceptable for older children. It also doesn't help that my local Target sells these movies in the Kids section. I mean you can complain about parents not reading the TEENY tiny warning label all you want but it's hard to expect parents to read the back of every "kid" DVD sitting next to Thomas the Tank Engine and Dora the Explorer. They have a kids section for a reason and if Marvel and DC TRULY believe these movies are for adults they should probably do a better job of selling them that way.