My transit company currently runs an ad campaign, based on a local show called "Lâcher prise". They want people to behave, i.e. not hold the trains' door open. One of their ad features the line:

*Lâcher prise, c'est ne pas tenir les portes.*

I somewhat expected this to read 

*Lâcher prise, c'est de ne pas tenir les portes.*

Kind of like "L'important c'est de ne pas...". What's the rule on the usage of *de* in those cases and similar cases?