It's not that they "aren't eligible," they would just have to get a waiver from their winter league team, which if my memory serves me correct would usually be granted if requested (merits of doing this could be argued). Eddie Rosario needed one from his PRWL team, for example, when he played in the AFL in 2013.
I took my information from the MLB web site about the AFL:
One foreign player is allowed, as long as the player does not reside in a country that participates in winter ball, as part of the Caribbean Confederation or the Australian winter league.
But, truth is, they have failed me before, as they no doubt leave out all the fine print. You are probably correct in how it works in practice.