User blog comment:Xandermcc/Arrow villians, not Batman villians/@comment-3221148-20161015102353

Aside from Prometheus, those characters are still predominately a part of Batman mythos (sub-franchises like Teen Titans or the Birds of Prey wouldn't exist without Batman seeding characters and purpose to those teams), which is why they get called "Batman characters" (even though Deathstroke, as you noted, is more of a Robin character, and had a rivalry with Green Arrow for one of the versions of how Slade lost his eye - that gets retold more than Joker's backstory).

The characters being international / having interstory appearances doesn't really mean much - that would also make Batman himself "not a Batman character". That's defined by where they originated, and where did they get the most of their characterization (in Deadshot's case that would be Suicide Squad, but SS itself walks on the principle of "gathering different heroes' villains", so Deadshot was considered a Batman villain by default). Otherwise, Batman himself wouldn't be "a Batman character", since he's a member of a number of teams with characters from other stories and countries.

Anyway, this shouldn't even bother you, since it's more of a joke of how Arrow steals Batman villains, just like BTAS stole Green Arrow villains like Count Vertigo and Clock King before.