From the Washington Post:
The roughly $80 million wagered on the NCAA tournament in Las Vegas represents a minute fraction of the total betting on college basketball's championship, dwarfed by the estimated $2.5 billion staked with neighborhood bookmakers, in office pools and, increasingly, over the Internet with offshore sports books based in the Caribbean and Costa Rica. The betting in Nevada's sports books also is the only legal way to bet on the tournament. And periodically, it's targeted by members of Congress, the NCAA and college basketball's most esteemed coaches, who feel it should be outlawed.
Betting on college sports threatens the integrity of the games, in the view of Bill Saum, the NCAA's director of agent, gambling and amateurism activities. At worst, it exposes college athletes to pressure from criminal elements conspiring to fix the outcome of games. At its most benign, it sends mixed signals about the propriety of gambling, whether on sports, slots, poker or pool.
The decision to shave points comes down to a balancing of its marginal benefits and marginal costs.