The way you're using "justify" here, makes it seem as if you think people feel it's morally legit to steal, if it's on a porch for... reasons?! From a moral perspective, theft is theft. There's no way someone can sanely claim they thought it was a free thing, because it wasn't locked away.
Doors and locks are there to make theft harder, more overt, loud, etc, and by no means validate when it's legit to be a vile thief.
Likewise, all spam is spam. The use of tools to make it more difficult for spammers to be spammers, is the same as having doors and locks. It makes it more difficult.
edit: What I said was, you clained they tried to justify it. So no worries, I was not implicating you.
I not trying to justify it, but if you actually look and check research about people who been caught stealing there is huge difference for them between stealing TV that dropped from a truck vs stealing from a porch vs stealing from inside the house even though it's the same TV.
Theft is theft, but for monkey brains there is huge difference between stealing someone wallet from a pocket vs picking dropped wallet and not returning it. So my point is that doors and locks work not because it's good technical measures, but due to how average Joe percieve social construct about them.
And for grey area activities online there is no such social construct because there is no percieved connection between bunch of email addresses and real people. Also in some countries it's totally legal to send you tons of physical mail spam.