The supply of software is not infinite. When you buy software, you aren't paying for the product you are delivered. You are paying the company for all the resources they sank into creating the product.
When he said "infinite" I think he meant that the number of copies is "infinite" in the sense that copying a program can be done thousands of times for a very low cost.
I agree with you about production costs and I buy all the software I use (well, the non-free software of course) but I think that this "easy copying" and DRM (to counter it) creates several issues on its own.
As I said before DRM often is problematic for real paying users while the pirates get to have a cracked version where the DRM is disabled. I think this is very unfair to the paying users.
I have several times chosen not to buy software because it used online authentication. This is because I change my main working computer roughly every year. If I have to get email contact with the author once a year about this it's irritating, but what if the author dropped support? What if it's a small garage company and the author had a car accident and died? What with all the work I've done, stored in a proprietary format using this DRM protected program?
When i buy a program/license I want to buy the right to use this program (at least the bought version) forever. If the seller does not agree on this I'm forced to chose other alternatives.