Enabling options that were available on a product and then selling it is very questionable, but I don't have a problem doing it for personal use. If it was ever calibrated, or serviced by the manufacturer you might run into trouble, like losing the options. You're bypassing a software lock, even if it's a weak one.
I don't have any problem with hardware modifications like making a 1GHz TDS744 (as long as you don't try to pass it off as a genuine TDS784). For one thing, Tek never sold a 1 GHz option for the TDS744, and never offered to upgrade their bandwidth after purchase. Your not trying to bypass keys, or other protections, you're just making circuit modifications. Then there's the matter of spending hours recalibrating it to 1 GHz, and there's no guarantee it will successfully calibrate to 1 GHz. Finally, any hardware modifications would surely void the warranty, which seems like a fair trade for any loss to the manufacturer.
It's like buying a car without the turbo option, and then adding it yourself. Just don't try to claim it has the factory turbo when you sell it, or expect the warranty to cover any damage to the engine. On the other hand, buying a car without GPS and then hacking the software to enable it is a different matter.