Oh, for gosh sakes, it's not to "reduce production costs" by a few cents worth of toner- it's the "give them a razor and sell them the overpriced blades forever" gambit.
But it's especially greedy of them to sell it with only a partially full $100 cartridge (or $500 worth of color cartridges), which carry the full load of hardware and are "chipped" to prevent refilling.
And they do reduce production costs-- by slapping together shoddy product from dubious raw materials in 3rd world countries, and not supporting it when the inevitable failures happen (in part by using other 3rd world nationals to waste time and money at first world rates over almost-free telecommunications links).
Only if you don't have an oligopoly, eh?