Villa were worse at the start of last season for having their subs running on the pitch. Liverpool were doing it yesterday and,as you say, annoying. However...I did look it up...law 3...it's only an offence if the player interferes with play. That presumes the player is not a sub coming on in which case they would commit an offence if they came on the pitch without the refs permission. I have seen a player booked for coming on the pitch off the subs bench and "interfering with play"....an ex student of mine, ex Villa player who got a bit too excited when Halesowen Town scored v Stourbridge in the no 9 derby. He ran on the pitch to celebrate and promtly got booked even though the ball wasn't in play.