This is a huge sign of confidence that they think they can do this safely and at scale... Freeways might appear "easy" on the surface, but there are all sorts of long tail edge-cases that make them insanely tricky to do confidently without a driver. This will unlock a lot for them with all of the smaller US cities (where highways are essential) they've announced plans for over the next year or so.
Freeways are easier than surface streets. The reason they held off allowing highways is because Waymo wants to minimize the probability of death for PR purposes. They figure they can get away with a lot of wrecks as long as they don't kill people.
"Easier" is probably the right one-word generalization, but worth noting that there are quite different challenges. Stopping distance is substantially greater, so "dead halt" isn't as much of a panacea as it is in dense city environments. And you need to have good perception of things further away, especially in front of you, which affects the sensors you use.