Sometimes, they're just plain dumb.
I disagree. They are surprisingly intelligent. The same deer that lays in my neighbors yard and looks at me in passing hoping I will feed it runs like hell when it sees me coming with a rifle. Point is they know the difference.
Also, consider this: deer are normally awake in the day and sleep at night. Yet in my area during deer season they modify that just a bit and nearly become nocturnal, sleeping either in our housing development or in thick brush in the woods during the day. Also, when I'm hiking in the woods during the summer, they don't care. Yet somehow in deer season they know to run. They are fully aware that there is a 'deer season.'
During hunting season I've seen behavior in deer that simply amazes me. Case an point: On more than one occassion I've seen deer in a herd 'take point' and actually serve as a lookout for other deer in the herd. Once I saw a doe (before doe and buck season in PA merged) stand at the top of a hill looking in all directions for almost a minute. The a herd, including a couple buck, emerged behind her and moved quickly across the hillside. Another time (during flintlock season) I saw a deer dash from thick brush onto an ATV trail, look in all directions and then dash back into the brush. I sat perfectly still, but at the point figured I'd been made--apparantly I hadn't. Just a few seconds later she re-emerged with a button buck and crossed the road into the area I was sitting. The second I moved, both ran.
Believe me, they know when deer season is...