There is a system in place that can prevent this sort of thing from happening. The guards, presumably, are paid to do a number of tasks and one of them, presumably, is to prevent WDW from losing money by people who are not staying in a Disney resort, parking in a resort's lot specifically to bypass the fee charged in the theme parks' lots. If someone is able to circumvent the system by waving a years-old pass in a guard's face, then either the guard isn't doing his/her job, or he/she has been instructed not to sweat it, which means this issue seems more important to us than it does for WDW. The only reason this bothers me is that, yes, a WDW vacation is expensive, and I can't help but think that some of the increase in prices, however statistically insignificant to an individual like one of us, is tied to losses they incur elsewhere, be it shoplifting or people complaining a meal was no good after eating more than half of it so they can try to weasel out of paying for it, OR parking in free lots and busing to the parks. Just like if you shoplift from a mall, it doesn't really affect me but for the fact that were they NO shoplifting (or no need to pay insurance for theft or no need for security cameras or extra security forces in the store or at the mall), the price of that item might be a little cheaper for people who ARE honest and who DO pay.
So yeah. It's Disney's fault, or at least the fault of the guard. Huan nature dictates some people are scumbags some of the time (others all the time, and others none of the time) If they did their job, then people would be less inclined to try these sorts of workarounds.