Why? We let you install it on two computers. Adobe doesn't let you do that with Photoshop.
Actually, Kirk, they do allow you to install Photoshop on two computers, as long as you don't use them both at the same time (the restriction for installing only one copy does apply to a copy bought with an educational discount though).
The only thing they don't allow you to do with Photoshop is install it both on Windows and on Mac as you buy only one of the two. Funny thing is, Lightroom does not have even this restriction.
Eventually we will add the ability to deactivate a computer.
Funny thing is, I didn't even know PS had a deactivation ability. If you implement such a thing, it would only be useful (IMHO) if you can deactivate copies running on other machines than the one you're using at that moment, otherwise you could not deactivate a copy running on a computer that has crashed. On the other hand, I can see this is dangerous too as this may allow people who abuse your license to block you from using your copy in a valid way. Ah well, nothing is perfect…
We instituted the activation system because Photo Mechanic is pirated quite heavily.
That's a really bad thing
Cheers,
Hayo