Actually, these rules don't affect whether or not it was legal before.
This rulemaking power is built into the DMCA, and don't have any retroactive effect that as far as I can tell.
These exemptions are only for a limited time of three years. Assuming it was illegal before to jailbreak, it is would now be legal until the exemption fails to be renewed. However, actions could still be filed on jailbreaks from last week, for instance.