The way they've framed this is not that it's a new rule, but rather, a statement as to how they intend to enforce the existing rules that are already on the books, and a "recommendation" to regulated entities as to what actions they should/shouldn't take in order to not suffer negative enforcement consequences (in other words, it's not "the rule is now that you must do this," but rather "just FYI, our interpretation of current rules/statutes/whatever is that behavior X is already prohibited, so if you don't want to get in trouble with us for failing to comply, you really ought to do this").

This is advantageous to the agency if they can get away with it because new rulemaking involves a bunch of extra, lengthy process under the Administrative Procedures Act (they have to publish a bunch of drafts and collect public comments on them, then address any substantive comments they receive, etc.).

Should allows for someone who has a real exception that we can't even think of to explain themselves. Maybe such a thing doesn't exist, I can't think of it. But just in case you have an out.