You're right that heatsink greases aren't necessarily dielectrics, but most of them are made that way because they're so commonly used around high voltage, such as for CRT TV or monitor horizontal output transistors, and zinc oxide, the most common additive put in heatsink grease, is dielectric.
By the way, nobody should assume that a grease is dielectric simply because an ohm meter says it is because that's not at all a valid test. Even Honeywell Tradeline aluminized grease will show infinite ohms, despite it being labelled as electrically conductive.