Presumably because you're a citizen, they can't actually refuse you entry because they missed their morning coffee. Those of us who work legally in the US, but aren't citizens, have a different experience. Sometimes I feel like reminding them, that I too pay for their salary.
Why is that to be expected?
Because when something happens over and over and over again, you tend to expect it to keep happening.
Because the US takes immigration and police in general too seriously. Everyone is a criminal. Nobody is to be trusted.
Compare this to Europe (minus the UK and France), where people in positions of authority treat you nicely until they have a solid reason to do otherwise.
"Cultural difference" or whatever.
Of course, US Immigration/Customs is not nearly as nice as immigration in other countries, but that's to be expected. (When I visit Norway and Denmark, the immigration people actually smile at you. When I visited Germany, the immigration officer actually laughed at something I said. It's like they are real people or something!)