This is a prime exampe of Wolverine not being allowed to maim or kill characters. Wolverine is a big deal for Marvel and they sell lots of books, movies, toys and various other merchandise to children off the back off his popularity. They won't have their cash cow decapitating others because a) Marvel comics are for kids and that level of violence would be too much, b) Wolverine would be killing or cutting off limbs in every fight and it wouldn't do to have a crossover with Spider-man, Thor, Captain America etc. and then having to portray Wolverine's foes in their own books and future appearances as having missing limbs or being paralysed from a severed spinal column, or dead.

It's not just Wolverine that has these limits on violence and its consequences. Torch and Havok, for example, have taken plenty of shots at other a-list characters, but have never really taken anyone down by burning or evaporating them. Strong chaps like Hulk or Wonderman are limited in what they can achieve. When they punch weaker opponents like Wolverine and send him flying, the effects on the human body would be catastrophic. It's part suspension of disbelief but it's also that such consequences would mean the death or disabling of numerous characters (Wolverine might physically survive a punch, but his brain would liquify from the shock of impact thusly leaving a brain dead Wolverine).