Work-to-rule is defined as a job action in which employees do no more than what they are bound by their current contract. By doing so they must also ensure that all safety regulations are met as to not make the action illegal. This in turn causes a slowdown in business rather than striking to serve a purpose.
To answer the question as you pose it in your example: yes you can "force" an employee to work overtime so long as it is not unreasonable, and that they are being compensated accordingly. Overtime (in Ontario where I live anyway) begins at 44 hours/week, at which point compensation increases to time and a half. There's dozens of examples and exceptions but to go into that would take away from the point of the thread....
Yes, there is an expectation for an application of some effort. In a work-to-rule action, the screeners are exhibiting lessened effort than what is common-place for the position, given the circumstances. If there is average traffic, with average security in effect, and the flow of pax going through the screening process is significantly slower, then it's obvious the screeners are doing something they should not (breaking a court-order in this case).
Regarding the questions in your final paragraph: I don't know the terms of their contract, so I can't speak to quantitative obligations, but there was clearly an observed deficiency in the quality of their work. I would assume they refused to work overtime (as it would contribute to an overall slowdown), although not wanting the overtime pay takes some real commitment!
I hope this answers any questions. My point is that this is not an issue of bad management, as management plans based on precedent. The precedent here was broken by a select few screeners who chose to partake in a work-to-rule action.
Flying refined.