See, but the situation with Harry was different. Harry is the only known person to have bounced an AK back at the caster (granted he had some help from Lily's magical protection, but still). Voldemort went in there with the full intention of eliminating his prophecized nemesis, not to make a Horcrux. The fact that he did was just a fluke, I think. And I don't think it was a true Horcrux, but some bastardly variation of one. Harry is a biological being; that in and of itself is bound to cause different magical circumstances than a diary or goblet – and it did. The piece of Voldemort that attached itself to Harry enhanced his natural abilities, gave him new ones, and allowed him a mind connection with Voldemort. (What a feature!) Sort of makes you wonder how powerful Nagini wouldn't have been without the added boost from the Horcrux.
To your other point, I absolutely see it as being an intentional thing that you'd have to think about ahead of time, and possibly prepare the vessel beforehand, as you suggest. Creating a Horcrux is a specific type of magic separate from the AK; otherwise, Tom Riddle never would have had to ask about it or learn it (and then, yes, everyone who killed someone would be unwittingly creating one).