These laws are enforceable, just not consistently enforceable. For the average "anonymous" transgender person out and about in public life, the use of their correct restroom will rarely be challenged. Other people are not usually paying that much attention and there is often enough uncertainty that if some one is suspicious, they simply walk away fearing they may be mistaken.
However, there are transgender persons who must use restrooms in situations where others know the person's circumstances. There is where some one can and often does take it upon themselves to enforce the law by putting up some kind of a stink. Cases like the transgender girl in a school that where her prior status is well known. Or a government worker that has come out and the coworkers know his or her prior history. These situations are the ones that can cause some of us such grief.
I live in NC. Right after the passage of HB2, my wife was concerned and while traveling within the state, I did an androgynous look and used the men's room. After the initial shock wore off, I went back to my full feminine presentation and freely use the women's room. For me, as an anonymous person, that works just fine. It may not work so well if I was a college student and had to use the rest room day after day in the same time slot around the same people since some one could very well catch on and take offence. In such an event, I would not have a leg to stand on.
Stephanie