Salters Steamers in lock, engine running

[Cashbuyer]

Apologies if this has been covered before, but why is a Salters Steamer allowed to leave it's engine running in a lock?

 

[Actionmat]

Do any of them turn their engines off? I can't say I've noticed any passenger vessel being asked or obliging.

My dad use to tell me that it was bilge water coming out the back. I didn't realise it was exhaust until I got my first boat