The Turbulence Switch
It's an airline truism that the seatbelt sign switch doubles as a turbulence switch. As soon as you turn it off, the turbulence begins. If you make a PA announcing that "we're expecting a smooth ride today," you can expect the bumps to be doubly nasty.
This switch carries a lot of liability with it. If a passenger is injured in turbulence, the first question the captain will be asked is "did you have the seatbelt sign off?" A few of the captains I fly with never turn the seatbelt sign off for this very reason. The thinking is this: If a passenger really needs the lavatory, they'll get up regardless of the switch position. That's true, many passengers disregard the seatbelt sign altogether. That's fine when it's not that bad and the captain is being overcautious, but the passengers have no way to distinguish between these times and when they really need to be seated for safety's sake. Always leaving the sign on might cover the captain's posterior, but it's not desirable for safety. Besides, it's not good customer service.
Many captains leave the seat belt sign up to the non-flying pilot. My own policy is to turn it off whenever we're in obviously smooth air above FL180, or if we're in light chop that shows no signs of getting worse. Whenever worsening turbulence forces me to turn the sign back on, I'll make a PA stating the reason that the passengers need to stay seated. On a few occasions, I've heard passengers entering the lavatory during solid moderate turbulence (it's right behind my seat), so I made PAs to tell the passengers that "you really need to stay seated with your seatbelts fastened right now."
A related "CYA" vs. customer service issue relates to the flight attendents. On flights over 30 minutes long, they'll do a snack and beverage service by default. If you're expecting moderate turbulence over the course of the flight, you'll ask them to remain seated and make a PA announcing that there will be no service on this flight due to turbulence. Of course, this is one way to ensure a silky smooth ride. The more common route is to tell the flight attendants to remain seated until you're climbing through 10,000 feet, at which time you can evaluate the turbulence so far and tell them whether to serve or not. The last thing you want is to hit moderate to severe turbulence while the FAs are pushing a heavy serving cart down the aisle.
Of course, the pilots can't always anticipate when it's going to be turbulent. I remember one of my flights from a few months back where there were no PIREPs or AIRMETs for turbulence. We had a smooth climbout from Portland until we reached FL230, where we hit a sudden band of clear air turbulence - moderate bordering on severe. My first thought was "oh crap, I'm sure we have a hurt FA or two." I called back to check on them. Luckily, they hadn't begun service yet and were still strapped into their jumpseats. If it'd been a shorter flight, they would've had the serving cart in the aisle already. The other thing was that I'd been just about to turn the seatbelt sign off.
Most turbulence can be seen and anticipated, but clear-air events like this do happen periodically. It's the reason why most airlines ask that passengers keep their seatbelts fastened while seated, even if the seatbelt sign is off. It's cheap insurance against a very real risk of serious injury.