They probably do as all US carriers do and dispatch with legal fuel plus a bit for a minimum hold time. They did it to me all of the time and if the hold was longer you had to divert to an alternate, not because you were running out of fuel, but because with the extended hold meant you didn't have enough to keep holding until the runway opened up. It is totally safe to go to your alternate because the company is trying to save money by not carrying extra fuel. This happens maybe once every two years per crew and the company thinks it saves money. The only time you have too much fuel is when you are on fire. I also know what it costs to ferry fuel. I have hovered over TGU in Honduras for over an hour many times trying to land with no nav aids operational in a 757 and got in every time because of the extra fuel to land in VMC conditions. Each flight requires specific fuel planning logic.