Another reason to leave gates like you found them/keep them shut, is that a lot of rangeland out west will only grow grass once a year. What I mean by this is if it's grazed accidentally in June, it won't really grow back by September even if there is above average rainfall. So in addition to losing a day or more to rounding up animals, the rancher has lost the feed source that would normally be available later in the year (if it's even theirs to graze, if not $$$ will trade hands). This also means less feed available for the deer/antelope/elk/etc.. that live there the rest of the year.
One would also be wise to remember that a herd of cattle could cost anywhere from the price of a decent car to well into the seven digits; if you wouldn't leave your dad's/best friend's/wife's fancy car in an open garage where it might get stolen please don't leave the gates open/different than you found them.