Printers probably use more ink keeping their heads clean in inkjet printers than they ever do printing these dots on the page. And even if it is expensive the customers bear that expense and send it right back to the printer companies. However many microliters of ink a customers' printers use is just more ink bought from the manufacturer.

But why? Why spend the engineering hours on this? In a world where any corners that can be cut are, why is this one not cut?

Is there a shadow regulation in place?

I'm not sure but again this probably required very little time to implement. The printer already has to process the rendered image adding an extra layer probably took a day to hammer out the first versions.

It's also potentially useful for business customers too to be able to where a document was printed from without central print dispatch and tracking.