29.99% Interest Rate Should Be Illegal

I’ve been keeping track of the transactions for my fathers Tractor Supply account, which is where he locally gets feed from. The balance hovers at around $600, and this is the account I’ll be tackling next once his $5k loan is gone. I’m making sure he doesn’t get anymore late fees, which are $39 a pop, by the way. I was looking through the transaction history, and I noticed a pattern in that every month his interest rate would go up. December it was 22.16%, in January it was 26.16%, February was 28.9% and now March is <strong><em>29.99%</em></strong>. I’m not sure I understand why it has the ability to raise so often, though I’m sure all creditors have the ability to do this. But to raise significantly every month? Maybe I’ve just always had a different type of credit account, mine have never raised, and I’d pitch a bitch-fit if they did. I’ve tried to google what this is called and why it happens, but have had no luck so far. Seriously though? 29.99% interest rates should be illegal.