no, they don't all eat plants! i asked the same question on this site a few days ago, and was referred to a very good website (i think it's the same one scott linked to). turns out some species of apple snail eat plants, while others eat only algae and rotting leaves and will actually starve to death in the midst of a heavily planted aquarium if they don't have enough algae. the thing is, it's hard to differentiate between the species. you can't tell by shell color, because many different species are the same color, so you have to tell by shell shape.
the best way to tell is if you have the kind of apple snail with a shell that spirals out into a point or spike (golden apple snail), it probably won't eat plants. i put this kind of apple snail in my aquarium, and it hasn't touched the plants, but eats the algae on the glass and gravel. if you have an apple snail with a spiral that lies almost flat against its shell (channeled apple snail), it will eat plants. i've heard most pet stores don't label them correctly, so you have to examine their shells carefully.
hope this helps!