And any would-be license that isn't already stamped as open-source by OSI or as free software by the FSF is an instant red flag, simply because there are plenty of licenses available to pick from.
> Many people consider copy left licenses more restrictive than your average commercial license anyhow.
Even though some people might say that, that's only because they don't know what they are talking about. A license for proprietary software does not give you the right to fork. As a recent example for why that is important, ask the people that have invested in FoundationDB how they feel about that, then read the story of MariaDB on Wikipedia to understand the difference.