[dirtboy]

I wonder what the legal basis for this ruling is?

It appears that they are issuing an injuction. From the AP article:

A three-judge panel of the U.S. Circuit Court of Appeals for the District of Columbia issued an order that blocks the rules from going into effect until the legal challenge from the states and cities is heard, a process likely to last months.

[inquest]

I wonder if "injunction" is the right word. Usually an injunction is an order to refrain from doing something (such as enforcing a particular law or rule). In this case, this appears to be an order not to refrain from doing something - that is, an order against not enforcing a particular rule. I'm not sure what exactly you'd call it, but you seem to know more about how these things than I do. This is getting to be over my head.