I'm totally guessing here of course.
But from a marketing and PR point of view, I'm sure Apple would love to be able to say that the watch, under normal every day use, should have a similar battery life to your iphone.
That may be totally unrealistic, and of course, it's going to vary with use, and of course we have the initial play with my new toy factor which always kills batteries on anything new we buy.
But I feel that would look and sound good from a marketing standpoint.
Naturally, like anyone else, I'd love the Watch, the Phone and the iPad to last AGES, but we're not there yet, and not looking like we're going to be there any time soon, unless something amazing comes out the woodwork unexpectedly.
Given the size of Batteries, esp on the iPad, Apple could I suppose offer what Samsung has, and allow the much smaller watch battery to be charged direct from another device if you wished to do so.