If you are crowded with potential customers, the guy trying to sell you a product you don't want is not where you are going to spend your attention. They also often are very slow to get the hint.
I'm not at all arguing that they aren't annoying. I'm saying that from their perspective, it seems like a reasonable choice. Cheaper and not stuck at a booth. If you're crowded with customers, then you're probably fine with a booth, but roaming is probably a better solution for the people whose booths are empty.