Be that as it may (or may not), she apparently did pay for the burger. Maybe there was no tax added since it was to go. I can imagine doing the same thing. I probably would have said "I changed my mind" and then the staff person could have added tax (or not).
In Ontario, there is no tax on take out food. The HST ("harmonized" sales tax) is 13%--so it might have been a couple of bucks. They say the employee was "disciplined"--for what, doing her job?