They can but what possible reason would they have for wanting to
i can only see two uses for this, lost children being found, and stolen/lost/dropped bands being found neither of these are a negative to me
you cannot get any personal information of the band (except whatever name you put on the back) only a unique number
that number is only relative to the Disney main computer which records all the information same as it has always done for your key to the world card
They may use it to go, oh crap 30,000 people are heading towards its a small world, lets try and get some character greets over there, have someone giving out fastpasses for other ride etc, and get the cast members ready for it
they are not going to go oh look john and jane doe from small town Indiana rode tower of terror three times, then went on rock n roller coaster at 11:35 they bought 2 mickey ice cream bars, and then went to watch beauty and the beast, because that would be a waste of time and money
You are right about lost children and watching how people as groups move through the park, but you are wrong about the rest. The certainly will note what John and Jane Doe rode and where they went and what they bought.
This will be extremely valuable information with which Disney will learn the patterns and desires of every possible demographic. They will learn that X number of people from X that spend X amount of time at Disney do A and B but not C. People from Europe do this, but people from Florida do that. People with X number of children within X and X ages will ride these rides but not those and will buy this but not that after.
They can do two things: Begin to change to better meet desires, change where things are placed in the park, change the offers made to guests, etc.
They will also be able to do this:
After their return home John and Jane receive the following email/letter/call:
"Hello John and Jane Doe, thank you for your recent visit. If you book a X number of days within this period of time we'll give you 2 special fast passes for each member of your party to Tower of Terror. Also, here are some coupons for free mickey bars. See the attached ad for the new Tower of Terror T-shirt, too! Thanks for your visit and see you real soon!"
From a business and statistical standpoint this technology will be a game changer. No longer will they need to do flawed surveys to find out in very general terms what you do and what you like. They will have it all in their computer system for evaluation.
Disney will be able to adapt to the consumer in so many ways it will make our heads spin. In a good way. Other companies can only drool over the advantage such information will provide!
I'm at Disney, I could care less what they know, because I know they will use it to improve the experience and save money. Good for us and good for investors.
It will be a monumental change to the industry. It probably already puts Disney 10 to 20 years ahead. They will make back 10 times the cost just in licensing to other companies.