"Compared to other illegal activities they might have engaged in such as selling drugs..."
The author didn't really assume they would be dealing drugs, he just wanted to show the advantages of their current scheme over another common illegal alternative.
Also (strangely) carries the assumption that these guys can't be multitasking and looking for other moneymaking opportunities, which might or might not include dealing.
a: if they weren't doing this, they'd be dealing drugs.
b: dealing drugs is the worst of all possible activities.
Interesting scam none the less though.