...with proprietary hardware/software. Some of the large commercial buildings my company does work in have fire alarm systems that can only be serviced/repaired by the company who did the initial installation. Often times, those companies will sell the original system for a very low price to get their foot in the door, knowing that they'll have decades to rake in huge profits on service calls, maintenance, and remodeling projects. I recently talked with a technician who used to work for such a company (Simplex) and he said their sales office used to have a banner that said something to the effect of "Get our panel in the building, and we've got 'em by the balls!"

It seems to be an effective business model. It's also effective at insuring that everyone hates you.