Astitchintime
As we have a water meter we have several water butts........none have ever been cleaned or disinfected. We have no desire to drink from them and they are only ever used for watering the garden after which we wash our hands anyway. The 'rotation' of the water is copious enough to not cause any stagnation so don't see the point.
Even daughters dog isn't allowed to drink the water.
The problem isn't from drinking the water.
I think the problem arises if anyone uses a hose, especially with a spray attached, running from the water butt.
When contaminated water is sprayed, Legionella can be transmitted from the water in aerosol form then by being breathed in.