I get annoyed at this at malls and other places, too. Especially when two or more people will spread out so that no one can get past them as they walk so slowly. Truly annoying.
I got annoyed at this this weekend WHILE HIKING. A group hike of about 40 people was moseying on trails in Rock Creek Park (complete with a guide, camelbaks, and those ski pole walking sticks). Ridiculous. Most of the trails are like 2 miles long. You don't need any equipment, you don't need a guide, and you don't need to walk 2x2 so I can't pass you without going off the trail and looking like a psycho.
In their defense, the people were very nice and offered to let me "join the hike."