They dont have the right, and there are millions of dollars being made every day by people who dont have the right to do something, but they do anyways.
So here are your options:
#1 sue them and gain nothing even if you win.
#2 Go pay them a visit and probably get arrested.
#3 find out why they are selling more of your design than you and fix it.. costs almost nothing.
Do you have the right to produce and sell every decal on your site? No.. So Karma came back around.. ignore it or fix it and move on.
on another related subject...
Anyone wanna purchase zombiehunterdecal.com zombiehuntersticker.com and zombiehuntershirt.com?
all 3 package deal $100 right meow!! lol