You're focused on the contract and abuse of the network. We're talking about what verizon can determine. If you tether without using a lot of data, is it their policy and practice to track people down? How do they do it? Would a proxy/encryption prevent this? The contract is just a piece of paper. I think we're trying to figure out what Verizon does in practice.
I can only conclude you have very little experience with the legal system if you believe "a contract is just a piece of paper." On the other hand, when this issue has come up in the past on this board I've noted that Verizon would be crazy to try to track down the occasional "tetherer" even if they could.
In the first place, as already noted, they don't have to "prove" that you are tethering your Droid; they only need to decide (not prove) that you are materially affecting other users. And downloading massive amounts of data fits that criterion nicely.
In the second place, VZW isn't concerned with tethering per se. They're concerned with demand on their 3G network that tethering may encourage. To that end, it's far more cost effective simply to examine the amount of data traffic on their 3G network than to examine how that traffic is triggered.
If, in fact, VZW has begun to look more closely at the issue of "tethering," they will simply look at accounts where large volumes of data are being consumed. The first step undoubtedly would be to remind a user of their contract provisions. The second would be to place additional charges on the monthly bill. The third would be termination of service and a black mark on the customer's credit. Problem solved from VZW's point of view.