Trust and respect, which is a 2-way street. I've bought some relatively expensive apps (the pro version of nearly everything Omni Group makes, Things, etc. etc. etc.) and all of them let you install and use the apps on all of your computers. They're licensed per person, not per device. I despise technical controls on this for the same reason I despise DRM on physical media: it's an inconvenience to rightful owners and a temporary speed bump to pirates.

I'm not about to abuse my OmniFocus licenses, even though I could. They sold me a great product at a reasonable price, with permission to throw a copy on everything I own so I can use it no matter which chair I'm sitting at. They trust and respect me, and I trust and respect them.