I was studied media in college so the answer above pretty much explains. You will not only see this in concerts, but in tv shows/reality programs. Alot of times you'll see a Sprite can with the "S" covered up with green tape. Or Instead of they'll cover up the "corn" and leave the "flakes" visible.
This is all about money. If you don't pay to let your product be seen on TV, then you don't get the priviledge of having your product be seen on tv. But alot of times it is in a musicians endorsement contract that when they play out, they have to have thier logo brand showing. Other than that producers are not going to let company's get free advertising.