There are multiple things you have to consider.
1.) The truth is there are gusts out there that know the rules of self sticks outside of WDW for places like Museums, but decide not to follow the rules and we don't really know how many people are aware of the rules as a result. Case in point is I witness a guess taking out a self stick on Main Street back in early September. We don't how many people were able to get a selfie stick past where you enter the park it between the final days of June of this year to last week. I can't say what I witnessed was a fluke before Disney tightened their security more.
However, not withstanding what people should do, we all know they don't. Otherwise people wouldn't complain about not being able to get walk up ts reservations. They wouldn't say things like I didn't expect it to business on Christmas day. They wouldn't complain about how using fp+ isn't advertised, they wouldn't complain about being unaware that they couldn't stay in the park for hard ticket events without purchasing said tickets etc etc.
There are still significant numbers of people who think you can turn up at Disney without doing any planning (mostly 1st timers to be fair) & those same people wouldn't think to see if there are any rules they ought to be aware of before they arrive.
Also having the selfie stick warnings up once you've got to the park entrance, even before security, doesn't leave you with many options as most people won't go back to their cars or hotel to get rid of the offending item & lockers to store them are only available after security, to the best of my knowledge.