Some thin because of the space the plants need to grow properly (you can't have two carrots right on top of each other grow correctly). For beans I'd say it's more of a jungle problem.

I planted my bush beans two seeds to a hole because they were leftover from last year and I worried a bit about the germination rate. Well, of course they all came up. I'm leaving them as is. If you decide to thin them with them that close, cut them off, don't pull them out. You will disturb the roots of the plant you leave and it might set them back a bit.