It's becoming increasingly common with this kind of hybrid product that has both hosted and on-premises offerings. You either trust your customers or you don't. And if you want their business, you need to trust them. Sure, some will take advantage of this - but most will do the right thing.

In addition, the customers that have requirements for this kind of offering are the last who are likely to skimp on payments. If a customer wants on-premise solutions, they do not want it calling home. Period, for any reason, not ever. These tend to be more regulated environments, where product licensing is taken seriously.

[edit: object and predicate should match. or something like that.]