I think it's very sensible to communicate the possibility of turbulence to the passengers for obvious reasons and to apologize for any missed service. That should be a management mandate. No-one wants FAs to get hurt, but also people do pay for an expected level of service (delivery of which is also a big part of the airline's job here). So if part of that service falls flat, you want to explain why and give your apologies.

The safety stuff is necessary but if you understand how customer service works, you know the need for positioning something like that. Good communication is key as per usual. Lack of communication causes suspicion and distrust. (Already plenty of that in the relationship between UA staff and passengers it appears).