Honestly, the ruling did strike me as a bit arbitrary.
But there's so many reasons not to care. Like how often does this ACTUALLY come up? 95% of the time if you don't want to gain anything, you just don't play the Smugglers. In the cases where you DO (Golem, blind draw off a TR'd TR) it's very unlikely that (A) your opponent gained a card from somewhere other than the supply last turn and (B) they also gained an undesirable card from the supply.
In the tiny fraction of games where this ruling is an issue, unless it's tournament play, you can choose to adjudicate it however you want with your friends. So, and I think DXV would say this too, in the grand scheme* of things, this ruling effects no one's life very much at all.
*Grand Scheme of Things- Action - 5
+1 Card
+1 Action
+$1
+1 Buy
At the start of your Clean-Up phase, you may place an Action card from play back on your deck.
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You may not buy this card if you have any Coppers in play.