Your observation is right in general (but note that the situation may be a bit different if some big-shot gives a plenary - people may take pictures or may even ask for autograms).
If the conference does not have an official policy, I would suggest to try to ask the speaker beforehand. However, as explained in the comments, speakers usually are happy to send you their slides (or upload them somewhere anyway), so there seems to be little use in taking pictures of moderate quality of some slides while you can have the whole presentation in perfect quality.
As a matter of fact, at some of my first conferences I did take pictures during talks. This was when I was new to the community and not to record the slides but to help me remember who the speaker was and to identify them in the coffee-breaks even a few days later. I made sure that taking pictures did not disturb the talk in any way (e.g. no flash and all sounds turned off).