The point is that nearly all of the people doing this don't even live in the country where the land is being sold from. A simple sign would probably be quite effective

True, but you can still do a confused deputy attack. The fraudster hires a property manager, informs them that they would like to remove the sign because they wish the list the property for sale. Either that or they con a realtor they're working with into doing it. The unknowing realtor, eager for the commission, knows a guy who can take it down.

For those like me who didn't know: https://en.wikipedia.org/wiki/Confused_deputy_problem

There's always something that can happen in any scenario. Social engineering, hiring locals, deeper forms of identity theft, or worse. The possibilities never hit 0, they just become a lot less profitable (and a lot riskier) a scam to try to run.

Yes, locks aren’t there to prevent the determined thief. They are for the 99% of other opportunists that will move on to an easier target immediately when they see your lock is harder to defeat

Who's paying for it? Are they working for free?

No, but paying someone $300 is cheap when you hope to get a check for several hundred thousand in a few months. (even if the scam is only to get the earnest money that is still a $300 investment for the final thousand or two you make - with very little work)

That's a lot of work plus money transfer paper trail for something like this.

Presumably the money trail leeds to the Caymon islands or other country where they won't assist investigation.

The realtor might pay for it or even do it themselves. It would take 5 minutes with a reciprocating saw. Or the scammer tells the realtor "never mind that" and the realtor tells the buyer.

The idea is just to avoid being the softest target. The scammers attempting this fraud don't want to do all the work you describe. They'll just move on to the next vacant property.

I'm sure you could put an ad up on craigslist or fiverr or whatever, one asking for someone to take photos of the property to see if there is a sign, and another to remove it. There's plenty of people willing to do anything for money.

Note that in the article, the author says how the scammers do everything to avoid having to show up in person. That's because they are in a different country and try to commit the scam without setting foot in the US.