I am sure they would weasel their way out of it by claiming that their staff are employed by ltd companies not 'self employed'
If I was a director of a legitimate Ltd. company I would negotiate my own contracts. I would then set the T's & C's under which I would provide my services. I would then invoice for those services to my customer. I would not let the customer have dictatorial control over my services and remuneration. I would not work for nothing, including unpaid SBY's or positioning costs & time. I would not pay for any customer demanded training and if required to work in customer demanded clothing would expect the customer to provide them. I would not be paid for holiday, except out of profits from my company, but I would decide when I took holidays. I would be able to work for any customer I chose, as & when. If any extra work was involved, such as an extended day I would invoice for that time. There would also be a compensation cancellation clause if I was on site and the work cancelled. Oh, there would be adjusted rates for night & weekend working.
In other words I would control how my Ltd. company functioned and how it charged it multiple customers.
Surely that is how it happens, isn't it?