Roles are not "permanent". They have a contract with agreed upon terms. If they want to lock in exclusivity for some time period, they can negotiate and pay for it. It's a transaction. Unless there are mutually agreed upon terms, the opposing side saying they want something doesn't make you planning not to give it to them unethical. If they had a contract term about this they'd be suing people, not whining about being treated unfairly by college graduates.