It's not that uncommon.
A company I quoted for a number of years ago wanted to know what our turnover was as they wouldn't take anyone on where their outlay was more than (from memory) 10% of our turnover.
If you think about it, it is sensible from their point of view.
It avoids the issue of employment if there's definite other work in the cleaners business.
And if an office or something relied on a cleaner every morning, say, but the cleaner was off ill for even a day-it can send the wrong example to their customers.
We clean a car showroom where they used to have a lady in to clean but if she was off sick it just didn't get done.
They couldn't operate like that.
So they called us.
I have a pair go in every morning (weekdays) to clean but if they're off sick or on holiday there are other people as back up and back up for the back up so it always gets done no matter what.
Though it does seem unfair to the one man band it does make sense from the company's point of view.
However... I'd never ever disclose our turnover to anyone at all...except HMRC & the accountants.
I'd never want a job that much.
And don't want to 'compete' with the 'big boys' anyway.