That's what I refer to them as!
As far as the US populations of giraffes go, Masai are the only pure population. All of the Reticulated and all but a very tiny handful of Rothschild's are all hybrids with each other and are managed as one population by the AZA. Most zoos call their animals Reticulated despite this fact, regardless of which phenotype they take after. Some still use both names, though, despite actively breeding them together. Bronx does this, for example, as their bull is a "Reticulated" while all their females are "Rothschild's". As for pure Rothschild's, I know that the San Diego Zoo Safari Park still has a few pure animals mixed in with their their hybrid stock, the B. Bryan Preserve's bachelor herd is all pure, and the Audubon's Species Survival Center has a couple pure animals. There may be some more left, but I don't know where they are and they're undoubtedly die off before too long.
As for the original question posed in this thread, I think the reason the US in particular tends to use Reticulated as the standard giraffe is because that's the name and phenotype most often used by US zoos throughout history. Wikipedia even lists Reticulated as being the most common giraffe in zoos, despite none of America's animals being pure and Rothschild's being the most common in Europe.
~Thylo