That's https://en.wikipedia.org/wiki/Selective_enforcement, which can be a problem, especially when contracts reference the policy.

To clarify your comment, this is exactly what OP was referring to with

> someone -including you- could then actually sue us for unfair business practices

That doesn’t really change anything. It’s still their policy to change. They still decide when to review the policy.

Changing your policy to be more friendly to all users moving forward isn't selective enforcement.

To reiterate, "policy" is just explicitly specifying "that's just the way we do things around here"

They can certainly change it, thus changing what they can do, in the future, but they can't make exceptions. A policy, written down, becomes a contract as far as the law's concerned. Contract changes are fine (provided all parties involved are then allowed to cancel the contract), but contract violation is not.

This isn’t accurate.

There are plenty of policies that state that they may be changed by one side or that exceptions may be made in cases where the wording doesn’t cover the intent.

The Apple store policy for example says this.