Because possibly they carried on drinking and it appears the PM was there for ten minutes. That's not a party defence, just reports from people who were there and have told the press that fines took place.
The PM (if accounts are true) instigated the drinks and the official photographer took photos.
It's a complete mess.
Wasn't it an offence to have organised a party/event though? He starts the party off, then leaves - so gets away with it again!