At first thought I agreed and thought it sounded a bit strange 'AI referring to AI' but as Giggity put it, it could be calculated in a way that decisions are something like 95% successful, to allow for mistakes and realism. You could go further still, and give individual stats for umpires, e.g Simon Taufel has a 95& success rate, and Daryl Harper could have a 70% success rate, along with the 3rd Umpires who would in turn have success rates, or as giggity suggested again 99% success rate with all the evidence and technology they have.
Implementing the UDRS is no different in my humble opinion to implementing the fielding or bowling. Why would the AI drop a catch, or why would the AI bowl a bad ball or play a stupid shot, because the players have stats so they can make potentially realistic mistakes for the sake of realism. The umpires are no different.
Whether or not LM wants to do this is another matter entirely as it is not doubt time consuming and arguably may not matter as much as expressed by the fans above, but it is a feature none the less that could be looked into