Open source is not a business model (2018)
Kind of Meta: The meaning of "business model," in a software/startup context is somewhat quaint:“All it really meant was how you planned to make money.”"Model" seems grandiose for a word that could be simplified to "income source." Comparisons to restaurants tend to be strained. Restaurant "business models" are not generally that complicated: People pay restaurants for food. Some deliver. The reason software people contemplate business models and restaurateurs don't is that "software," regardless of licensing, doesn't come with a standard business model. Every one is a snowflake.In software, "business model" is a wide range of possibilities. Most of them failing. Many quitely succeeding. A few explosively winning. Is AirBnB a software business model? Is Adwords?  Even MSFT, which does literally exchange software for money... MSFT's "business model" is a complicated, impossible to replicate thing.The majority of software that's literally exchanged for money is done with some sort of services/support bundled in... and the customer is a business. 
If I give it away then why do people buy it?" is not even a sensible question. What does that question mean for Google, Apple or Amazon? There are plenty of exceptions: Photoshop, MS office... but even these are becoming online services and the "business model" is less directly "software for cash."TLDR, software doesn't have a standard business model regardless. Weighted by revenue share, the vast majority of software revenue comes from advertising, luxury electronics, web services or some other "business model" that isn't contradicted by OS in any way. "Selling software" in a sandwich-4-cash comparable way is surprisingly rare, regardless of license.