Making an app and releasing for free it can be fun. When the customer support and feature requests start pouring in it becomes a lot less fun. Chances are more likely that a project will be abandoned for one reason or another (developer gets bored, gets busy with their 'real' job, etc.)

When people see an idea that's good, especially for potential business use (and especially on HN which as a community of entrepreneurs) they are letting the developer know that they have created something that is worth having around and would pay to see it maintained.

Linux is probably a bad comparison, because it has so much corporate support and is an entire ecosystem of business models for probably hundreds if not thousands of companies.

Why are you afraid of open discussion about pricing?

(Millions of dollars have been invested in Linux development by commercial interests, even if most distros are free-as-in-beer.)

I think it was just out of curiosity; I had the exact same question.

Also, "Why is it free?" is another way of asking "What's the catch?". It's important to understand this before you put in a bunch of work...