It is not just to give them a way out if they close the flight early, it is also for possible oversale situations. When that happens, UA offloads the misconnects at the dot of T-30. Needless to say, the misconnects are reacommodated, but no IDB payment and no IDB black mark with DOT.

It's fine to board latish in the process so long as you are around, but it's a risk to schedule a connection. ZRH only has one service/day. While there are alternatives, they are less convenient.