Paying for Bug Fixes in Their Own Code

I have a long standing relationship with an outsourcing provider in India. Things have been going well with them billing for hours worked. However, I see now that at least 50% of the time being billed is due to bug fixes, or improperly coded implementations they built. Should I be paying for this?

My argument for "yes" is that if they did it correctly the first time, it would have taken longer, and also, bugs are a natural aspect of programming and need to be addressed as part of the process.

My argument for "no" is that the time spent on the bug fix is usually a lot longer than if programmed properly in the first place.