I thought it used to be in the LOTG as a caution, but couldn't find it. I may have been thinking of the old US Advice to Referees, which included it as an example of USB. (It may have been in the old IFAB Q&A, but I'm not ambitious enough to look back that far.)
IMO, it is often cautionable if it rises to the level of needing to be called. Actively trying to block a punt is a real knucklehead move that can be dangerous and can be designed to intimidate the GK.
I think timing of action is a critical element in deciding if what the player did was a foul in the first place--did he move to block/intercept before the release (likely an offense), or after the release (not an offense)?
The judgment on offense or not on this is similar to evaluating the actions of a player who has not yet retreated 10 yards when the opposing team takes a FK--if that player waits till it is kicked to move and intercept, there is no offense, but if he moves forward before it is taken and intercepts it is generally an offense.