> Is it less-ethical to have provided software as open source, and then later become a proprietary product? Why?
Because usually these companies use OSS as a marketing gimmick, not because they believe in it, or want to contribute to a public good. So, yes, this dishonesty is user hostile, and some companies with proprietary products do have more respect for their users. The freedoms provided by free software are a value add on top of essential values that any developer/company should have for the users of their software. OSS projects are not inherently better simply because the code is free to use, share, and modify.
To be fair, I don't think a developer/company should be expected to maintain an OSS project indefinitely. Priorities change, life happens. But being a good OSS steward means making this transition gradually, trying to find a new maintainer, etc., to avoid impacting your existing user base. Archiving the project and demanding payment is the epitome of hostile behavior.