Different MLS actually have different rules about what must be in the MLS. In Oxnard the majority of the homes are usually listed with the Ventura Regional MLS. In the MLS database, there are several areas where the broker/agent can enter that the home is a short sale or short pay (same meaning). There is also a remarks area for information that other agents showing the home need to know. The Short Sale information is usually listed in one of those areas as a contingency of the sale, just as the Seller reserving the right to have a home of choice or a concurrent closing escrow. The remarks that you see using a service that uploads MLS data, such as Zip or Realtor.com, is only allowed to upload certain fields, per the individual MLS's contract with that service. So, you see thing that are important for marketing a home such as the marketing remarks that sometimes say that the home is a short pay. This is the agents individual choice to determine if sharing that the home is a short pay will entice the buyer to call them thinking that they might get a deal on a preforclosure. Marketing remarks are very subjective and as long as the information isnt unethical or lacking in compliance with fair housing rules, there is a lot of choice left in the hands of the agent that writes the description. For example we've all seen the work "sparkling" or "beautiful" used in a home description but it isnt a fact, its an opinion. So the field that you are seeing "short pay" written in, is a fact but it is also a subjective choice of the writer.
I can really understand your frustration but I have to say that if you were working with a Realtor in your area you would save yourself a lot of time. A Realtor can access the MLS database directly and send you only listings that are not short pays, if it is your preference not to see any short pays.Plus a Realtor can help you with so many other areas of your real estate purchase. As a buyer you may not know that most often the Buyers Agent, the Realtor who would represent you, is paid by the Sellers Agent. So, I think you should consider working with a Realtor who can help you with the entire process and aleviate a lot of frustration.