I can speak from experience regarding hockey players. Hockey players don't really have "agents". They have "family advisers". They aren't paid and the players can't sign a long term commitment with the adviser. The advisers often become agents later on, but subject to the same rules as football or basketball.
The reason for the exception is that 18 year olds have the option to turn pro in hockey and go the minor league route or Canadian juniors. The player's family can use these advisers to sort through the pro vs. college process. A couple of my former players had these family advisers.
I suspect it is similar in baseball since there is also the option of pro straight out of high school like hockey.
the anti-agent rule is because the agents ahnd the players money as an "advance" against future contract money.
Thanks, that's interesting. I suspected that was likely the case.
Interesting how differently people's opinions on this matter are when the sport in question generates revenue for the school.