"Or if there's an open-source clone of your company's software, surely there's nothing stopping your company from, er, competing with it?"
Writing a copy of something is easy. Creating something new and original is hard. Any decent programmer can write a Tetris clone in under a day, yet the original probably took a much longer time to think up, prototype, design etc.
Same with other FOSS knockoffs (like Frozen Bubble) - the hard work has been done, all you need to do is make a copy of the working commercial version and you have some freedom at a fraction of the effort required by the original developer.
Make your own unique software, sure, but don't pretend ripping someone off is somehow fair competition. Come up with your own ideas - as all your free clone will do is kill the original development and thus cause the project to stagnate as there will be no more reference designs to copy.
It's the FOSS version of Embrace, Extend, Extinguish.
It is easier to change the specification to fit the program than vice versa.