Can the Army force you to buy Unit T-Shirts?

No they can not make you purchase a unit T-shirt, but why would you not want to purchase it? Purchasing that t shows unit pride and cohesion are you not proud of your unit and don’t you want to be part of the team. If the answer is no to either of those you my friend have joined the wrong organization.

When I was boot csmp the second time around my company gave everyone the impression that the unit t-shirt was required for the fun run. I and many others simply ignored this and faced no repercussions.
I don't know what your situation is, but that was my experience.

No they can’t but they can make it tough not to. So stand your ground and don’t buy it. You shouldn’t have to conform to their demands. While they are wearing their unit shirts you wear a tie dye shirt. That will show them! Sometimes it’s just easier to conform. You fall on your sword at every fight and you will bleed to death.

Leaders who see the unit morale value and then require (more of less) the soldier or Marine to pay? Somehow, leadership got lost in this. As the MOI at an NROTC Unit, I wanted my drill team to have the "unit morale" value so I paid for the t-shirts, the screen printing, and the individual marching positions on the back. It was a strain as a Captain stationed in LA, but asking/requiring college kids, or soldier or Marines, (especially the "college program" midshipmen who were not on scholarship) to pay? No way, Jose. If it wasn't important enough for me to bear the financial burden, it sure as hell was no important enough to make Midshipman Glotz (or Spec 4 Batelli or LCpl Baseplate) to pay. I applaud the 1stSgt who worked a way to make them available to all without financial burden on any but the CO and him. That, folks, is LEADERSHIP.

Be a grown up, quit sniveling, and buy a shirt. Buy 5 shirts. If you're like..., everyone else, you waste more money than that every month.

You're over thinking this. Buy the shirt. There will be more important battles, choose them wisely.

Good question and failure will keep you from ever making E5. Unit T-shirts are for morale and spirit de corps. I've benn retired 20 + years and I still wear most of mine with pride. Do you really have the intestinal fortitude and low mentality to tell your 1SG/CO, CSM/LTC that " I don't have to buy it!" I never ever recall anybody intentionally looking for a reason to get on my shitlist!

Dont buy the shirts from those commies save the money. How can they track you down to harrass if you dont have on the unit shirt? Plus that 10 or 15 bucks is a lap dance and beer.

At WIC many years ago we were highly encouraged to buy class T shirts. We all did willingly. Easy to tell which class a candidate was in based on T shirt color. Blue class, gold class, and the best for last red class !

like has been said, pick your battles. When I was in the ARNG, it wasn't "mandatory" that you belong to the NG officers association. However, it was very thinly veiled implication that failure to pay your dues would reflect poorly on your decision making ability on your next OER.