I don't think this is true, my fathers employer had a 3rd row box with 10 seats, and everytime I went it was only the 10 people sitting who were ever in the box area during intermissions.
Most companies give them away to employees or clients, I can't see why they would sell them when they could really boost employee moral by giving them away. This is of course, assuming it's a large company (like my dad's was unilever), and not a small one.
Also, a lot of the time they wouldn't even use the box area, instead just giving the individual tickets away for use. The expense a company has to take to open the box area is ridiculous, and can usually run up several thousand for one game.