I figured out that if I open the link in a private browsing tab, I could just vote, and vote, and vote... Not that I did anything like that of course.
Since it appears (from comments in-thread) to likely just use a local session tracking cookie, it would be a damn shame if some unscrupulous person(s) were to, say, run a simple for/while loop from the shell on their pc to politely try the link (get/post) every 30 seconds or so, then flush any cookies and wait to try again...
How do you vote incognito? As is, I'm limited to 6 per day with a PC and two laptops and two browsers.