There is a difference between assuming that they are expecting you, and having your enemy know your every.single.move.as.you.make.it.

EX: If I'm going to punch you in the back of the head, I'll sneak up behind you. You might know I'm there, you might not.

Now, if I come up behind you at the same speed while yelling, "I'M COMING UP BEHIND YOU AND I'M GOING TO PUNCH YOU IN THE BACK OF THE HEAD," you'll definitely know I'm coming, and I should expect a countermeasure.

That's pretty-well the difference there.

Expecting, yes. Watching your every move in real-time, in 1972, no.

Remember, this was in the early days of television - not the continuous video/metadata surveillance state we have now, with commensurate capabilities available to any idiot with a motive and few hundred bucks. Sure, the police assumed the terrorists were expecting a raid somewhere between right now & hours/days from now and involving a lot of heavily-armored police with lots of firepower, but wouldn't know which second and the exact arrangement & capabilities of forces involved. Even seeing all the TV cameras around, the police hadn't learned to make the instant & automatic connection between a video crew and the TV set on in the hotel room about to get raided. This was the first time the targets of a raid were able to see that there were, in fact, right now, 12 cops standing just feet from the windows, and 5 approaching the front door carrying machine-guns and about to bash in the door with a battering-ram in 7...6...5...4...

You should expect it and plan for it, but when you know it your actions have to change. If I know you're about to kick down my door, I have far more options than if I know "in the next hour they'll probably kick down my door". The information completely alters the dynamic. Even in games with no hidden information (such as chess or go), you don't know what my next action will be (except in few cases where my options are severely limited), you only know the probabilities. What if we're raiding a house for some reason (like in Munich), and there are 4 doors and a dozen windows. 5 gunmen inside, if they know which doors we're about to go through and which windows we're ignoring, they can either effect an escape or a more effective defense. Our raid is going to be fair less effective if we announce our plan to the opponent instead of making them guess at it.