Probably profit oriented. If you sign up for 6 GB and don't monitor your usage and adjust your plan each month (like most probably don't) and you only use 3 or 4 that month, they make more money!
We always start out with a low data bucket and have notifications sent as we hit 50, 75 and 90 percent. If we get a 90 percent usage notification, we increase it immediately. It's a bit cheaper to increase the monthly allotment before you go over your monthly amount, than to pay the higher overage charges.