Actually, (no sarcasm), this was a major hole in the clearance adjudication criteria up until this past April.
Here were the (pre-April) guidelines within the area of "loyalty:"
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Note points 1-3 talk to "association or sympathy" -- but point 4 only talks to "involvement."
In other words, up until this past April, you could say whatever the heck you wanted, but unless you were caught doing something, an adjudicator would have been powerless to characterize the person negatively.
Here is the same section that went into force this April.
Note that "association or sympathy" is now a factor with any of the factors.
The big point is that prior to April, they couldn't consider irresponsible Fakebook posts, Tweets, and so on (public statements like on social media could demonstrate "sympathy"). Now if a person expresses "sympathy" with a violent group (like BLM, Antifa, etc), that is grounds for a far deeper look at the person, if not an outright negative adjudication.