Because if they disable it with software, they still have to pay the royalties or whatever that they are paying for each system. I agree with you that they shouldnt have taken it out, but they made a determination that they could save some money because some people wouldnt want it, and if tesla installs it in each car, they have to pay the license fee or whatever, whether its "activated" or not.
Personally, I believe it should be a click box you can check on the online order, so it comes factory installed if you want it, and not if you dont, but tesla is trying like mad to not have any real customization to a specific car for a specific person. People think when they order a tesla they are getting a "custom order" car, and they absolutely arent. They are just waiting for one with their spec to be made by the factory during its regular manufacturing.
This is different than, say, ordering a BMW, which you can order pretty much exactly how you want it, with several options etc, and is actually built for "you". I only mention BMW as its the brand I have the most familiarity with, having driven pretty much only BMWs for about 16 years or so before buying this model 3.