The reason for the agreement is to protect the CHL so it can keep their players in junior instead of in the minors. It is not only designed to keep NHL teams from sending CHL eligible players to the minor pros, but is also designed to keep the minor pro teams from raiding CHL teams.
Without the agreement, minor pro teams from the AHL, ECHL, UHL, etc would be free to sign undrafted 17,18,19 year olds from the CHL, which would be crippling to major junior. Imagine an 17,18,19 year old undrafted player who is playing for his major junior team and he gets a phone call from a team in the ECHL wanting to sign him to a contract. Most undrafted players would take a pay cheque from a pro team rather than play for almost nothing to play junior. These 18-19 year old players are needed to keep the CHL competitive enough so that NHL can send their prospects to junior because they are not quite ready to play at any pro level. There is no harn in keeping these players an extra year or 2 in junior because if they are good enough to play pro at 18 or 19, they should still be good enough to play pro at 20 years old.