It's not normal. It is very very unusual. I found Karwa taxi drivers to be polite and honest at all times.

I agree with businessman. You probably didn't notice the airport pick up extra, which is added up to the taxi fare not only in Doha airport, but in any airport around the world.
First time out of your country?

I never had any problem with Karwa taxi drivers. The problems with Karwa is that they (its managers) are unable to meet the transport requirements for Doha's residents. There is also a huge problem with its booking system.
Of course the fact that they pay a real low wages to their drivers doen't help.