100g is a lot for the type of drones used there, it's comparable to their whole battery pack weight. Also, the power draw of such a transponder is likely to be significant.

Sorry, but unless it’s a toy drone, the battery weighs a lot more than 100g. I own almost 20 drones and I think only the Tello batteries clock in at that range in weight and it’s useless imho to a police department. My Phantom (nearly 500g battery) / Inspire (515g) / Mavic (~250g) batteries are all way heavier than 100g and the batteries I use for my S1000 are over 2,000g. That said, 100g additional payload is significant for everything but my S1000, but I had to correct the statement re: batteries.

As to transponder power usage, it would be very negligible compared to the power draw of the ESC/motors.

First, even if the transponder weighs 100g mounting it securely on the drone will likely add more weight and affect flight. Ditto for reliable receive / reply.

Even if it were equipped with the transponder it is not a given that F-15, as flown, would be (A) interrogating it, (B) show the pilots the replies, (C) show the pilot the replies early enough to make the difference. Pilot flying F-15 at 500AGL is likely training for other things than avoiding small drones. I think at those altitudes and speeds he is given the clearance to exercise (500+kts; not the regular restriction of under 250kts below 10000ft) in a well defined region and assumes that it is clear.

Would it make a difference though? I don't think F-15s have TCAS / would respond to a RA.

Some of the RAF Tornado GR4s have TCAS; there's an ongoing safety concern about them colliding with civilian aircraft or each other, especially after two collided near Lossiemouth with casualties.