Upon further examination, it turned out the victims had all fallen for the same scam -- they found unopened drinks inside the machine's dispenser slot, presumably left there by the last person, for whatever reason. Since the human brain tends to shut down all further thought upon the discovery of free shit, they scarfed down the prized beverage ... then quickly regretted it. The poison was so effective that victims would often start to feel sick before they had finished their drink. We're guessing they still finished it, though, because hey, it was free.
To be fair, there was actually another, ingenious layer to the poisoner's strategy: The product most often tainted was a Japanese energy drink called Oronamin C, which was running a promotion whereby vending machines would occasionally dispense a bonus second drink. It wasn't hard to imagine that some customers were walking away without realizing that a second drink was rolling down the chute, a phenomenon that the killer was taking full advantage of -- the unsuspecting victims had even less reason to be wary.
Otsuka Pharmaceutical Co.
Being sold a brutal, unforgiving onslaught of cute-n-quirky probably didn't help either.