It's not a suggestion, it's a fact. BSD is permissive open source software, not copyleft free software.
Copyleft has never been a hard requirement for free software. If anything I feel like it stands against the deeper philosophy of given no restrictions on usage to the end user. I understand that's not the RMS-take, but he's not the only voice in this community.
> [The FreeBSD license] is a lax, permissive non-copyleft free software license
https://www.gnu.org/licenses/license-list.html#FreeBSD