Product: Credit card or prepaid card

I have a Guitar Center store card that is now being managed by Synchrony Financial. I have had the card for about 4 years. All the purchases that I have made on the card have had a promotional 0 % interest period for several months. I realized several months ago that the total balance on the card was much more than the 0 % promotional balances. I called the company to find out where this other balance was coming from because it was not explained on the statement. They told me that the balance was from 2 different transactions that were not paid off by the expiration of the 0 % interest period. All the payments that I had been making for the 18 months since the 0 % interest period expiration were not paying off ANY of these balances. ALL the money was going to pay off other transactions that were still in the 0 % interest period - which of course allowed this somewhat small amount of about {$600.00} to continuously accrue interest at 24.4 % rate for the past 18 months. Also, there were several purchases that I had made that had 12 months or more of a no interest period that were paid off several months early because any amount over and above the minimum payment went solely to balances NOT subject to interest. This obviously allows Synchrony to keep the balances, with high interest rates, high so they make as much money as possible. While keeping the consumer in the dark about these practices. 
Also, there were several late fees of {$38.00} charged over the course of the last couple of years. When the late fee was charged it was added to the minimum payment which I paid in full each month. However, I was just informed by a Synchrony account manager that I have a balance of over {$200.00} in late fees. I said that is not possible because I paid them every time one was charged. They explained that they DO NOT allocate the extra {$38.00} that I paid to pay the late fee -again all the payment money gets distributed to the 0 % interest balances. This is not explained at all in the statement. In fact the first customer service agent I talked to had to go digging for several minutes to actually explain to me what actually made up the total balance on the account. This is a HUGE RED FLAG. Everything on the account should itemized with detail if the company is being truthful. 
Also, the statements and website gave NO detail what so ever on this rolling balance or this other balance of accumulating late fees. They do, however, give a great amount of details on purchases that are still in their promotional 0 % interest period including : the original transaction date and amount, how much of the previous payment went to that particular transaction, etc. This gives the illusion that I was in control of my account and that there was nothing being hidden when in fact the majority of the balance of the account, about 2/3s, is  never being paid off and is hidden with no itemization on the statement or their website. 
I found out about the rolling balance 3 months ago. The only way Synchrony will allocate money to the high interest rolling balance is if I call them every time I make a payment and explain that I want the payment amount to pay of the rolling balance and not the 0 % interest balance. I just found out today that part of that rolling balance is also the late fees. So, in the future if I have any late fees, I will also have to call them and tell them to allocate that extra {$38.00} of the payment to pay the late fee or, again, it will just accrue and be charged interest. 
These practices are extremely deceitful! Synchrony is stealing money from their consumers. I am also going to contact Guitar Center and explain that they have lost a customer due to their alliance with Synchrony. I will never shop at their store again. 
How are these practices legal?!