To understand the issue, you need to look at the "why" of the situation. The reason that WDW does the "finger scans" is to prevent ticket sharing. To accomplish this effectively Disney doesn't need a foolproof system that locks things down to the fingerprint level... or anything close to it. If they can use a system that only gives a 1% or 2% (or even a 10%) chance of Person A being able to pass themselves off as Person B then they've succeeded in effectively stopping ticket sharing. Disney's systems take length measurements of two of your fingers, drops those numbers into a formula and the result is a single number that's stored with your ticket information. That's it. Information about your fingers cannot be reverse engineered out of the number as more than one set of differing lengths of fingers can result in the same end result "number".
It's like the check digit on the end of a UPC bar code. If you look at a consumer package's bar code you'll normally see a single number set apart on the right from the rest of the numbers. That number isn't actually part of the UPC, it's the UPC "check digit".
Here's the explanation of how they compute the number. UPC check digits are used to help ensure that the barcode scanner read the right number. If the cash register looks at the full bar code read with each scan and sees that the read check digit matches the one expected based on the other digits read to the left of it, then calls the code "good". Since there's only ten different check digit possibilities (0 - 9), the system offers a 90% chance of catching a bad reading of a bar code. However, if they used a formula that utilized 2 check digits, the rate would go up to 99%.
Also, keep in mind that the more complex system that Disney uses at the turnstiles, the more the chance for problems and the more data "overhead" they will have to deal with. Actually fingerprint reading will take up more system storage and more processing time to compare. Since Disney doesn't need a system that 99.999999% accurate, they aren't going to make any more grief for themselves than they have to.
As to the OP's comments about the security guard's stated prohibition, photographers (pro and amateur) have been putting up with this sort of hassle from police and private "Barney Fifes" ever since 9/11. In almost every case, the official has been 100% wrong about the supposed prohibitions that they state. Unless your taking photos of something like a military facility protected by an Executive Order, the exterior of a prison, or something similar, then you're OK. And while it's true that the owners (or their representatives) of private properties and eject you from their facility if you do not honor their request to not take photos, that doesn't stop their guards from making up such stuff either.