Maybe it's the whole Fullmetal Alchemist thing of "In order to gain something, something of equel value must be taken." Such as Jolly Roger gained powerfull voodoo magic, but at the cost of mortality(or humanity, causing him to turn undead. you get the idea..
)
My question though about Amo Dorsi(found the name on a wiki..
) did he curse Jolly Roger in self defence? or did Jolly Roger kill the Witchdoctor in self defence(but was to late in killing the witchdoctor and so, was cursed.) But somethings that are lost can be found again. Would it be possible for Jolly Roger to revert back to being human(most likely losing his voodoo powers in exchange) Anyways, back to the topic. either way, his powers that we've seen him wield during Invasions? Could be possible that thats just a small itsy bitsy teeny tiny smidget of the voodoo powers he wields. Same goes for what happened to Bo Beck, probably just a sneak peak..
EDIT: Who knows? Maybe Jolly Roger lost his right hand and left leg performing Alchemy and had Winry give him automail: The Peg Leg and the Gun! :P (Wont get the joke unless you've watched Fullmetal Alchemist, Sorry.. ^_^)