The Law of the Land

I have a question.  When is the law of the land not the law of the land?

Answer:  When the president has a pen.

Last year during the government shutdown, the republicans wanted to make some changes to Obamacare such as delaying the mandate and the medical device tax as a condition for raising the debt ceiling.  The democrats would have none of that, declaring that Obamacare is the law of the land and could not be changed.  This was one of the reasons for the government shutdown last year the republicans had included those terms for raising the debt ceiling.

Now, however, it is OK to change the "law of the land".  With just a stroke of his pen, the president has changed this law not just once, but many times.  Can someone please explain how it is OK for the president to change the "Law of the land" and the congress cannot change the "law of the land".  It is their job to introduce, debate, change and vote into law the bills that originate in congress.

I just don't understand.