A MIT or Apache license might detract contributors, because essentially these licenses say "your contribution is OSS today, but it might end up resulting as unintended free labor that helps build a commercial product of which you won't be part of".
However some see GPL licenses as a guarantee that external contributions are actually contributions to the overall Open-Source community as a whole, because they cannot be turned into other non-OSS product without explicit permission of all people who built it.
So yes, copyleft is more restrictive than non-copyleft. But only in the subtle sense of ensuring that no further restrictions will get added by any third party, ever. Which is a strong promise that should encourage contributions.
Ultimately, my objective is to learn if this theory matches the practice.