Would a Cadet at USMA have to payback if he/she admitted allegiance to the Communist Party?

No repayment. His mental health, ability to function as an officer, ability to lead troops, and security clearance need to be re-evaluated. We have harsh assignments for officers who need reeducation and reformation of disordered thought processes and behavior. Guarding prisoners in Fort Leavenworth, Camp X-Ray Guantanamo - similar assignments. Warmest Regards, Sandy

Service academy cadets/midshipmen who voluntarily leave after starting their third year have the option of paying back or enlisting. The knucklehead Lieutenant completed West Point and received a commission, so probably not. If his behavior runs afoul of UCMJ, he can be penalized and potentially lose his commission, but his thoughts are his own.

Membership in a political party is not illegal. In the articles I read, I don't recall any mention of party membership. Just expression of some odd views. Odd in that it seems extraordinarily unusual for his chosen profession. In 1967, the Supreme Court struck down laws that made it illegal for members of political parties to be denied public sector employment, the right to vote, holding office, etc. It was aimed at laws making Communist Party membership illegal.

Holding political views and advocating the overthrow of the government are two different things. He published an article, so I think that it would be a stretch to call it subversive. It's not political activity. Federal law defines that as supporting a candidate for office or political party in an election. Reasons to pull a security clearance can be pretty subjective. The question to ask is does he present a security risk.

I think the guy is an idiot. I think expressing his views is unlikely to earn the favor of his peers and superiors, so he's probably not going to have a long career.

After commissioning they are subject to UCMJ and separation procedures just like the LTC that stole from unit funds or the LT that sexually harassed the unit driver...
Those in Mil schooling be it ROTC or a service school that fail to commission but were contractually obligated, can be required to fulfill their contract as an enlisted person.
I saw that done once in my career .. when I was a DS.
Part of a new platoon inception and integration was a filling out of personal data card and getting to know them a bit.. like what is your MOS.. I had an older male recruit that insisted he did not have an MOS, it would be assigned to him after basic..... That made no sense to me at the time..
Turns out he was a 4 year scholarship receiving tuba player that finished collage and got his degree and but refused to commission, so he was ordered to report for duty, where he would go though basic and an AIT yet to be determined and serve his 6 years of obligated service as an enlisted person achieving the the rank no higher then E4