She is still only 15. I understand some kids can handle the responsibility of cutting their own hair. She can not. So she isn't allowed to.
This comment is very worrying.
Is she not neuro-typical? Does she have mental or developmental disabilities that you have not mentioned?
Assuming she does not have any developmental problems, then why do you believe she is incapable of handling the simple responsibility of styling her own hair? Is it because she would make style decisions you would not approve of? Is it because she has come to you worried about such decisions before?
She may well call you crying because she knows you will be angry and fears your punishment, not because she regrets her "impulsive and poor" decision making. She may well have cut her hair on her own because she knew you would never support her decision to have her own hair her own way.
Unless she is atypical or has special needs, you refusing to let her make decisions about her own hair sounds overly controlling, stifling, and even potentially inhibiting. Your lack of confidence in her abilities, and refusal to allow her such simple and personal responsibilities, could cause her own lack of confidence in herself, and her own inability to rely upon herself. That cycle could be, in and of itself, the very reason why you don't believe she can handle such harmless decisions as her own hair. Encouraging her to make her own decisions, and to deal with (and learn from) her own failures, is the only way she will ever be able to grow and develop, as well as the only way she will ever prove to you that she is capable.
At ~15, it is inevitable that she will begin to pull back from you, to rely more upon her peer group far more than her parents and family, to seek the advice of friends and to trust their opinions (particularly about things like fashion) more than she trusts yours. That's because she is growing up, preparing to strike out on her own and become an independent adult, and learning to depend upon (and trust) herself and others her age. As adults, it's what we all must do, and as teenagers, it's what they all must prepare for. That is not to say that this Jack is harmless - he does sound like somebody you should keep an eye on - but she will be taking more advice and instruction from peers than family from here on. Frankly it's lucky she hasn't started doing more of these things already.
I simply cannot agree with "only" 15. She is "only" 15 in the sense she should not be given the responsibility of a credit card or a flat, but her own hair?? I cannot agree with that, unless she is disabled - and even then, it's only hair and it grows back.
As you are in America, it will be less than three years until she is legally free to strike out on her own, undertake binding contracts, live in her own flat or even take on a bloody mortgage, die in a war, get permanent tattoos, take out and use credit cards, support herself financially, and in all other ways become an independent adult. I won't even tell you what she'd be legally free to do over here just next year. Obviously loads of learning happens in those three years, but if you cannot trust her now to make such simple, reversible, and completely harmless decisions as her own hair, how on earth will she ever become responsible enough for you to trust her supporting herself and making ALL of her own (including very permanent and impactful) decisions as an adult, in only three short years?!
What she needs right now is your support and encouragement, not your discipline. It's past time she had the most basic body autonomy like a hair cut. Punishing her and continuing to forbid her making such personal, temporary, and harmless decisions on her own will only push her away from you at this volatile age, and she will run right into the arms of potentially bad influence peers who she feels enable her to have the freedom she so craves and needs.
(Yikes, sorry for such a long post! Brevity is not my strong suit!)