Why it doesn't fill from the top
I have never seen a waterer that does. It makes sense, though, since once you open the top, the vacuum would be gone, and all of the water would dump out.
It is not too hard to fill, you do have options of opening the plug, ot taking the bottom off. It would be nice if the bottom was easier to line up and put back on, but it is OK. In the summer, I open the plug and fill. In the winter, I bring it in the house, take the bottom off, and carry it out upside down, then flip it right before I go into the chicken yard, so if it spills it in outside, and not on the birds.
This one works pretty good, but when the temperature goes below zero, it will ice up a little. We had to break the ice on ours this morning.