The logic is the different demographics of the parks cause one system to work well for USO and not for WDW.
Numbers are made up, but due to USO's bigger emphasis of thrill, maybe 5% of the people apply for their disability service and it is granted This is manageable, and does not clog any lanes
Now due to WDW being more about theme and nostalgia, imagine that 25% of the people apply for their disability and using the same rules as USO have it granted... now you have an issue where 25% of your park aren't waiting in lines and are virtual queuing. This clogs restaurants, stores, walkways, ect. additionally the LLs are so backed up, guests paying to skip the line are not satisfied and are requesting refunds. Furthermore, some of the guests on DAS can not wait in the LL now because the line is too long for them to wait!
The policy could be the same, but the results are vastly different.