I would argue for a fourth point in this case

* Purchasing X through a third party allows us to circumvent laws and regulations meant to protect workers and/or consumers

These "laws and regulations" still apply to the agency that hires those workers. You're simply paying for them to carry the risk.

You can argue that such agencies are more likely to disregard laws and regulations, but how would you fix that? With laws and regulations?

If Big Publicly Listed Company A contracts with Small Fly-by-Night LLC B to provide workers to vacuum and empty wastebaskets, and it turns out that LLC B is illegally withholding wages, then Company A could be partially liable because they should have done due diligence and picked reputable law-abiding suppliers.

I can think of a couple different legal metaphors for this:

1. Something similar to banking's Know-Your-Customer could work... maybe call it Know-Your-Suppliers.

2. People are criminally liable for possessing child pornography even if they had no participation in the direct crimes of its creation. In other words: in this case there is no immunity just because the crimes happened somewhere "upstream".

And in practice, it just doesn't happen much.

For instance, how likely are your Fly-by-night employee or sub-sub-contractor to rat out the Fly-by-night? They may get a beating, their checks withheld, the Fly-by-night go under.

If it still happens, Big Publicly Listed company pays for it to go away in a settlement.

If it goes to court, they Big Company can say "we work very diligently to avoid this situation in the future and what happened is in no way indicative of our values".

Etc.

It is as simple as “X is available more cheaply from a third party supplier.” But in the case of services that depend on low-skill labour like room cleaning, there are no presumed economies of scale. These companies don’t get the same work done with less labour, like utility suppliers. Instead, they pay workers less, and use complex business structures and confidentiality agreements to protect the client from the consequences (unions, lawsuits and PR damage). This isn’t capitalism realising positive-sum efficiencies; it’s just taking resources from workers and giving them to business owners.