* Naive screencaps are much less traceable to the leaker than a naive photo is. Yes, someone can strip out EXIF data, but we've seen over and over again that they generally don't. And even without EXIF a naive framing on the photo is more likely to expose information about the location or identity of the person who took it.
* A photo of a webinar is going to (barring serious postprocessing) look much less official and be less legible than a screenshot, so the use cases for illicit captures are going to be fewer. Few people are going to try to take a phone photo of the top-secret meeting and use the slide in their next team all-hands, but many might forget the rules and than snap a screenshot really quickly for later use.
* Just having the ability to block the easy method of screen captures helps avoid cases where the person doing the capturing isn't actively malicious, just ill-informed. If a normal employee attempts a screenshot and is reminded they're not supposed to do that, they're not going to pull out their phone to take a photo, they're going to say "oops" and move on.
Yeah, there are threat models that won't be stopped here, but most of corporate InfoSec is wrapped up in protecting against pretty lame threat models that would benefit from this—mostly uninformed/ignorant employees screwing up without intending to be a threat.