Kansas City Times, June 23, 1918.
I'm not saying this sort of thing is ever justified, but anyone who's ever waited table for a living will relate to this particular crime at least a little tiny bit. Give the Dalai Lama a couple of busy lunch shifts and he'd dig it too.
The term "Mickey Finn" more typically refers not to emetics but knock-out drugs like chloral hydrate, which made it easier for unscrupulous service personnel to rob their clientele. The name commemorates Chicago barkeep Michael Finn, who ran a notorious State Street clip joint called the Lone Star Saloon until the law shut it down in 1903.
Ptomaine poisoning is bacterial food poisoning, so the wait staff was implicitly framing the kitchen while exacting revenge on the non-tippers. Again, a lot of service workers will probably relate.