Oh come on...
"placing the tea-cups on a busy roundabout is dangerous"...
True, but no more dangerous than crossing any other road.
"not just for the perpetrator but also for the drivers using the roundabout"
Ffs. A handful of mugs on a roundabout is dangerous to a driver? Why? Are they distracting in some way, more so than any other random roundabout decorations, or even a sodding traffic cone? A pair of jugs maybe, but not a bunch of mugs.
None of the above is an issue beyond the normal or everyday. Common sense and driving / crossing with due care and attention negates any real risk. The council could have come across as a lot less tight-assed by saying "Well, we don't get it but it's just a bit of fun. Just take care doing it and don't do it if it would put you in danger."