Re: I cannot understand the merits of this case
"Should "Rated People" which allows you to post some plumbing work that needs doing and a plumber can reply and take the job be liable for paying minimum wages and giving that plumber holidays regardless of other work they do or how many jobs they take on?"
If "Rated People" are setting the price for the job, then yes, they should be able to estimate the time the job should take and factor in the "wage" for the plumber, including his/her costs and travel. But since they don't, then no.
The same applies to AirBnB. The seller sets the price, not AirBnB.