And if you are allowed to do that and want to do it, why not? The purpose of a warrant is to verify that a judge has done due diligence and found the search to be justified, but if you check for yourself and come to the same conclusion, the warrant isn't really necessary.
But if you check yourself what. By looking at their data? How would that be okay? I'm not putting words in your mouth, just looking for a logical conclusion.
If you look at their transaction history with your service, it's not just their data, it's also yours and you have every right to look at it.
Now what's morally ambiguous is then deciding to reveal that data to others, and if you did it indiscriminately for all customers I'd be against it. But on a small scale, I don't think it's so wrong.