I am one of those people that will pick up disgusting plastic bags just lying on the streets and throw them away. First of all, it's gross (find your own planet to dump your trash on), but it's also dangerous for animals. A lot of this stuff just gets swept into storm drains, rivers, lakes, streams, the oceans, etc. And who ingests them? Marine life. One of the worst pictures I've ever seen is of a seal with its stomach cut open and a mess of plastic inside.
There are so many fronts on the war for animal rights (and I'm increasingly believing that "war" is the right word for it) and often times it's hard to know what to do. The protection of marine animals may fall under "environmentalism," but these are artificial divisions. Anything to do with an animal's survival or well-being, whether a species or an individual, should fall under animal rights.
Here's a short opinion piece on plastics and marine life.