I haven't run in to this often, fortunately. But I did study sales and representation, and I can tell you they're going about things the wrong way :P.

I think unfortunately, as others have said, some people are paid on commission or have quotas to fill and they fear losing their job. This is an impractical sales method which in the long run actually reduces the amount of sales because it leads to a pushy, unpleasant environment. Then people will not return, or will leave without purchasing anything because they feel pressured and uncomfortable. Sadly, a lot of businesses are short sited in this regard, and if sales drop they will blame the sales people, as opposed to the inefficient methods they teach them.