At the time, some of the NCAA's own enforcement personnel didn't think it was a serious enough violation to warrant the penalty that was imposed. So, OK, it may have been "illegal" at the time, but it was a first-time-offender misdemeanor violation by IU instead of a serious felony.
As for an "ADVANTAGE", as you screamed, I recall some of the "phone calls" the NCAA counted against Sampson were (1) not received by any human being, but (2) were registered by telephone company equipment when they clicked on answering machines for a couple seconds even though nobody left a recorded message. Again, not a serious felony giving an "advantage", and it's doubtful the caller or recipient even knew this would count as a "phone call" or that it even had happened. The NCAA counted "phone calls" even when they were not completed.
One thing is clear -- when the NCAA acts like this it loses public support. The NCAA's tremendous loss of power proves "Karma is a bitch."