There have been a few cases where Midshipmen have been expelled or refused to accept a commission after graduation. In those cases, they were ordered (and forced) to pay back the cost(s) of their education. Something in the neighborhood of $160K for the four years. The Navy (and the Army for their guys) considered that the student was repaying the tax payers, since it is we, the people, who pay for that education.
I agree completely with that idea. Having said that, in the fine print there's a line about "you agree to accept a commission, if offered". It was the same thing I had in my contract with the Army ROTC program. If you fail to accept, for whatever reason, they can involuntarily induct you for a 6-year standard contract as an enlisted man (or woman, as the case may be).
The Navy can also delay the commission, for a certain length of time, or have the individual also serve in the reserve component while awaiting a slot for active duty. They can also reduce, or lower the amount of time required for your active duty commitment. That's what happened with Roger Staubach, who was another Naval Academy graduate. He did 2 years active duty, including service in Vietnam, and then came back to play with the Cowboys.
I suspect that in Cardona's case, if he makes the team, he could have his active-duty time waived for 2 years, or even be released from his commitment altogether, depending upon the needs of the Navy.
It's going to be interesting to watch and see what happens.