See the thing is, we've had a country for about 240 years. And in all of those years, Congress has passed lots and lots of bills, many of which were signed into law by the president at the time.
Most of those laws never expire, and most of those laws are supposed to be executed by the executive branch, but more importantly, most of those bills delegate lots of the details about how to execute the laws to the executive branch. That's generally how laws are written everywhere.
So, in this case, as in pretty much every other case when dealing with executive orders, the president isn't just making up laws, he is changing how the executive branch will execute laws in ways that were delegated to him by congress. It - whatever it is - is perfectly legal because past congresses and past presidents made it legal (and a court has never ruled it unconstitutional). If the current congress doesn't like it, they should pass a bill to clarify the law so as to restrict the president's ability to interpret it in a way they do not like. Of course, as is built into our system of checks and balances, they have to pass that law with a supermajority that is immune to the current president's veto or get a sympathetic president elected for their attempt to mean anything.
That system works fine so long as unrelated items aren't put into bills that have to be introduced periodically, such as bills to fund the government or raise the debt ceiling. Congress could have chosen to attach this to the continuing funding resolution or the debt ceiling bill, and told the president to sign it or the government would shut down and the country would default on its debts, and then maybe an unsympathetic president would sign the bill. Of course that could also make it harder for those congresspeople to be reelected.