It doesn't sound like they're trying to get high, per se, just work harder without feeling it. A "recreational" dose would probably interfere with getting work done.
I know they aren't just "trying to get high". The problem is that in order to get any psychotropic effects from Tramadol, the dose is so high that a lot of other really nasty effects will come with it. A 5mg dose of hydrocodone would accomplish the effect they are after with very little negative physical effects.
If they were just using it as an analgesic, which the article clearly indicated is not the case, it would be a different story.
As someone who takes Tramadol for pain, it does have an odd effect when I'm tired: I find I become less tired, as if the Tramadol is making me ignore my fatigue. Oxycodone, on the other hand, causes me to be more drowsy. This might be what the farmers are talking about.
Second, the definition you are describing is a depressant, not a psychotropic.
If you think repeatedly taking extremely high doses of Tramadol is somehow safer than small doses of hydrocodone, then you're unbelievably ignorant.
As a side note, hydrocodone is one of the least potent mu-opioid antagonists, but it is more potent than Tramadol. That is the only reason I mentioned it by name.
And I take no stock in governmental drug laws with regard to pharmacology, so your mention of it being removed from medical use in a handful of countries means nothing to me.