I think there must be a Brit etymology to the bare naked “seen” without its “have” - lots of Brits, Aussies and Irish use it who seldom make other grammar faux pas which are that egregious.
That makes me feel a little better. I was visiting some of my British relatives for the first time in years and another American in my party, who shall remain nameless, used "seen" instead of "saw", several times. I cringed and assumed they would see it as a low class Americanism.
I've noticed that the "I seen" usage is very common in the Northwest US.