Do you know what Judgement does?
Lowers the Armor of targets hit.
Do you know what The Black Cleaver does?
Lowers the Armor of enemies hit with physical damage.
Do you know how Rammus works?
Redirects damage back to the attacker, scaling off bonus Armor.
Put all of this into consideration:
Garen builds TBC, and casts Judgement. The damage from Judgement doesn't proc Thorns, but lowers the Armor. When Garen is done with his spin, now that the Armor of his target is completely brutalized, he can slap a Trinity Force Q and silence on his Target without any punishment from the thorns damage.
+
+ 
OK
OH!! And btw Rammus used to have a counter for bullshit like this. His Puncturing Taunt used to lower the damage dealt by target. But not anymore.
Riot wants Rammus to be an assassin and it's bullshit. Especially since they nerfed his ganks and obliterated his mobility all together.
THEN to add insult to injury- Riot comes out with another update for Rammus after his rework...
What did they do agina??? Oh yeah- they made him dance faster the higher his Movement Speed...
[zombie-brand-facepalm]