It's simply the business model that they chose. What is unsupportable is that when they found out their business model couldn't survive the free market, they used the DMCA cudgel to try to support it with.
They are attempting to sell something, "ink" which I suspect costs almost nothing to make at an extreme profit. They attempt to do it, as you say by keeping anyone else from supplying ink for their printers.
It is a simple case of trying to have a mini monopoly.