bad snap on punt

[BigCat]

Fourth and 15. High school game. Home team is back to punt. Snap is grounded and punter simply kicks ball while it on ground rolling. it goes down field 20 yards. visitors run it back 10 yards. officials throw flag saying illegal kick. visitors decline penalty and they play on from spot visiting player was tackled.

i know nothing about football rules. seemed strange. i thought maybe it would be like a field goal attempt. curious as to if there was a penalty. there was a long conversation on the field. what is the ruling etc...