They are against it because it would destroy college football.
You think there is no parity in college football now? Just wait till the big time boosters can "legaly" buy recruits.
Then, what happens when a booster makes promises to recruits but stops paying when the recruit doesn't turn out to be a star? Will players start sueing boosters for breaking verbal contracts? Will they start demanding writen agreements from boosters before signing a LOI? If there are signed deal, that would surely effect a players amature status, but then again if payments from boosters are now legal that couldn't possibly hold up in court. So the NCAA would have to scratch the amature rules all together, because these kids are now being paid to play. So how long until an unknown kid puts up huge numbers at a smaller school, and decides he wants to transfer to a school with better paying boosters? How long until the unions come in and says, these kids are now basicaly employees of the boosters, and they have the right to unionize and negotiate a CBA so that all players get treated fairly?
Not to mention all the want to be agents that will be acting like boosters to get close to these kids, with the hope that one day they'll be able to truely exploit the kid. Then again it will end up a semi profensional league with most players having agents already. So the want to be agents will now be working on the HS kid to exploit.
Allowing boosters to legaly give players money or gifts would open up a Pandoras box that would be the end of college football as we know it. That is why most peole don't agree with it.