It's to prevent 1 person occupying the space provided for 4 people, under crowded conditions following delays, which is exactly when people want to sleep on the benches and ignore all the other people looking for a seat.
On a more general note, many airport operators, including my one-time employer the BAA, prefer to keep people moving around the shopping opportunities to purchase over-priced tat and "duty-free" stuff, mostly goods on which there is no duty in the first place. They achieve that by filling the space that could have been seating with more tat emporiums.
They also strive constantly to maximise the "dwell time" that passengers are forced to spend in their shopping malls, but that's another story. It used to be a Key Performance Indicator for terminal managers to lengthen the dwell time.