Why do Epsom insist you buy both black and colour ink?

Question

revealingly, their website doesn't have a comments/complaints section, but in any case they'd probs spin some yarn about the quality of the print. But why, when you want only to, say, print text in purely black ink, does the printer not work unless stocked in both black AND colour. They are not only expensive, they can also be hard to find, especially if your local shops do not stock them or you have an older or less popular model of printer. So I amnow waiting for another colour cartridge, which has cost me money, with a stack of work that cannot be printed.

Answer 1

hp does the same.
cannot win these days

Answer 2

because the printer will use a lot more ink and a lot faster if you are using just a colour cartridge and not a black one "because you need to use all the colours to make black" if you have a black one then the printer will use the black cartridge for printing black and thus saving your colour cartidge for colour items