How can a company develop software earn money if they use linux?
The same way you earn money developing for any other operative system really. Some commercial software are ported to Linux and it is sold precisely like the windows and mac versions. But since Linux evolved from Gnu and an open source movement, there is a strong tradition of getting things for free here. I think in the end it may boil down to what type of software you are referring to. I don't think that many users would pay money for a new text editor here but if you make 3D games then here is a market that grows every day.