Yes. CC licenses (and our messaging about them) make clear that they don't impose restrictions beyond what copyright imposes. Doing so would make them an instrument of evil (or at least arguably such an instrument). This doesn't mean that licensing nearby restrictions, such as sui generis database restrictions, is ruled out (this would still be a permission grant), but it rules out using contract to impose additional restrictions.
It is at least conceivable that this approach is wrong -- that imposing additional restrictions would maximize the value of the commons. However, one must consider the global costs of doing so, as well as evaluate the non-theoretical benefits -- and at present it isn't clear that there are many. If the concern is "leakiness", contract is a pretty weak plug against bad actors.