Good idea. Perhaps you should suggest it to them.
found that they could be easily fooled
Well, yes, and no. As has been mentioned, tickets purchased together can be freely interchanged. Also, they do turn off the scan checking from time to time---mostly when they need to get a big bunch of people through the gates quickly. Once you get a feel for how the turnstiles work, you can tell when they are on or off, because the turnstile unlocks almost at the same time the scanner flashes when they are off, but there is a perceptable delay when they are on. (Security people call this a "timing attack").
But, when they are on, you've got somewhere between a 1/1000 and 1/10,000 chance of getting a "false positive" for a ticket that isn't directly or indirectly linked to your reading---it's a verification scheme, not an identification scheme, and it's tilted in favor of false positives (allowing guests in when it shouldn't) rather than false negatives, for obvious reasons. We've tried swapping UCT tickets (so, they are not linked), and when the scan is actually being used, it's pretty good at spotting swapped tickets.
But, at the end of the day, it doesn't need to be perfect. It just needs to be "pretty good", because they just need to catch people with a high enough probability to discourage most people. Heck, even if they left the scan off permanently, but just went through the motions, that would probably be enough of a deterrent. While there is still a market for "used" tickets, it's a lot smaller than it was before MYW was brought into being; partly, that's because most tickets now expire, but I think this also contributes.