
In truth, it is not legal but they do it on the basis of logical reasons.
Their first reason for doing it has to do with the amount of fees the company is required to pay in terms of bank transfer fees or PayPal service charges.
The second, is that they prefer to wait a month before paying for a paper just in case the client decides to file a quality complaint or begin charge back actions.
They prefer to hedge their best by protecting the income.
Any penalties arising from problem transaction will be taken out of the writer's wages. Not the company share of the profit from a given order. It comes across as an insurance method if you think about it that way.
However, as a business person, or from a business point of view, I can understand why they would do it that way.
It is the norm of the business. It was that way when I entered into the academic outsourcing services and that payment method still stands today, long after I have left the business.