Intermediaries are often just an easy way to get out of liability. It can happen to small companies that you don't know about or are powerless to your supplier's illegal practices. With big companies you know and have the power, which makes it your responsibility and you being part of the problem.

There's also a trend to give big companies liability for their supply chains. Germany just enacted a supply chain law last year and there are talks about similar laws for the EU: https://en.wikipedia.org/wiki/Supply_Chain_Act