the breach of quarantine is that they clearly transit to/from outside of their residencies for no essential purposes.
If they were to do their works inside of their primary residency - this is allowed I would think. But driving to a shed each day in groups?
So it cannot be classified as outdoor activity for physical benefit, as solitary activity, as assisting someone in need (no one lives there as a primary residence), or what other essential exemptions are there..?
I agree with you that it may sour relations but this comes after messaging landlords politely explaining both how it's a risk in addition to inconvenience. Incidentally - if it prevents my freedom of movement under limitation (eg dog walk for risk of immediate contact with a transmitter right outside the house), then what? I guess no one cares although that's hardly fair.
I don't really see how it could be a health risk to you. But anyway, it's a free world and it's your call. I personally would tread very very carefully.