It's right there in the words of the term - it's not an unreasonable guess to think that 'open source' just means that the source code is 'open' in the sense of

It's right there in the words of the term - it's not an unreasonable guess to think that 'open source' just means that the source code is 'open' in the sense of 'available', especially since proprietary software doesn't normally provide source code. Compound this by the fact that many people are jumping into this field where English is not their first (or even second) language.