Because they offer a service. They do something for 'you'. Why should they do it gratis? Would you like to do work for someone gratis? And I think, teaching is work! Sometimes it's fun, but I'm sure, not always :)) I think it's ok that they are paid!!!!!!!

Because this is my profession, not my hobby.

Do you work for free?

After running an extensive and expensive opinion poll two main reasons emerged from my constantly humming computer room: 1) to buy foods and stuff; 31.4159 % of the sample also for growing up children. 2) to store the money safely in the bank. Both are good reasons to my eyes, on the firm ground that those money have been legitimately earned, but this last consideration is out of the question, isn't it?