the basic fact is that they don't need a reason to deny you.
It is always a game of Russian roulette when ever you apply for a visa or intend to travel their even if its just transiting somewhere else.
With the US, I think if they refuse a visa then you can appeal (it's how the US works). If they grant a visa and the immigration guy decides he doesn't want to let you in, you can ask to put your case before an immigration judge. If you travel on a visa waiver, the immigration guy can just say no and you're on your way out, no appeal.
Given the number of arrivals they process each day, I suspect the number refused is actually quite a small proportion.