DISCLAIMER: I don't own John Constantine. That's slavery, and slavery is illegal in my country. I just borrowed him. Sort of... Hope he doesn't mind.
It all started out as a joke.
A horribly macabre, but above all funny joke.
No one in their right mind would actually go through with it. It was disgraceful, possibly unhygienic, and probably illegal.
But it was funny, which is what really counted at that point in time.
As to the other reasons on not to do it, well when had he ever cared about hygiene and legalities? Never, that's when.
Which is why, to this day, John Constantine's body sits in a shadowy back corner of a pub. Locals think it's funny when tourists do a double take.