You _do_ have a point, but by your reasoning, the business model for most web apps is also flawed. Is e.g. 37signals' business model flawed? Should they not be able to charge for use?
Most web apps are hosted on infrastructure that costs money to maintain, and can't be easily be duplicated. When you pay for that, you're not paying for a product, you're paying for a service. It's analogous the difference between buying a lawn mower (where you're paying the company for a one-time act of producing a lawn mower), or paying someone to come by every week and cut your grass.
Personally, I'm not a big fan of the software-as-a-service model, but there is a difference.
