True enough. However, Apollo didn't curse her without cause. The deal was supposed to be that he gave her the power to see the future, in exchange for her being his lover. He gave her the power, as he had sworn to do---but she reneged on her half of the deal.
He couldn't take back the power, but he could make her a horrible example of why doublecrossing one of the gods was a very bad idea. She's one of the earliest examples of someone who tried to bargain with a supernatural power and ended up regretting that she hadn't read all the fine print first. If she'd come through with her half of the bargain, she'd have had a divine protector and people would have believed her prophecies.
So, while the Greek gods can be arseholes, I do think that Cassandra had it coming.