Seeing a lot of confusion about this, so here's a quick clarification: CBP's authority does NOT stop at 100 miles from the border. CBP agents are immigration officers whose powers are delineated in 8 USC 1357 1/

Section (a)(3) notes agents have the power to "within a reasonable distance from any external boundary of the United States, to board and search for aliens any vessel within the territorial waters of the United States and any railway car, aircraft, conveyance, or vehicle..."

This reasonable distance is not defined in the statute, so it's defined in regulation. Specifically, 8 CFR 287.1 (a)(2) defines it as "100 air miles from any external boundary of the United States or any shorter distance which may be fixed by the chief patrol agent for CBP."

The Supreme Court has upheld CBP's rights to operate with enhanced authority in this zone, including stopping and searching vehicles on some pretty sketchy evidence. HOWEVER, this doesn't mean agents don't have jurisdiction outside this zone. They just don't have special powers

They are perfectly capable of otherwise using their standard law enforcement power to conduct arrests or, say, fly a drone over a city

It's still pretty odd for CBP to be using a drone to apparently assist a local police department in monitoring a protest, but as far as I can tell there's nothing legally preventing it (could be other impediments, but the 100 mile thing doesn't come into play here in my opinion)