Eventually they’ll come up with a solution to solve the problems of guests visiting, staying or those with reservations having parking issues. Maybe they need to build more parking or re route conventioneers somehow. I don’t know the lay of land around the convention center much, I just see that this has been an on going problem, something else I was not aware of.
Their solution is to limit cars to those people who are guests of the BWI/BWV and conventioneers. Conventions pay Disney a ton of money to use their space, their attendees are going to expect parking at the resort. (And conventioneers often have cars because - hey! expense account! I was always surprised how many of my coworkers would want rentals in cities like Boston or London - where they are FAR more hassle than they are worth) The other part of the solution is to provide Magical Express for free, which does make a dent in the number of cars at Disney resorts (imagine the issues if everyone coming into the airport rented a car!)
Parking ramps are VERY expensive to build - Disney isn't going to spend money on a parking ramp so people can buy a $10 ice cream cone or a $14 martini. And they certainly aren't going to build it so someone can get validated for a $10 ice cream cone and avoid Epcot parking fees when they are staying offsite.
And Disney wants to limit the number of cars on property. Cars are involved in accidents - accidents on vacation are not magical. More cars means more accidents. Plus, with a car you can get to Universal or Sea World. There is little upside to Disney in encouraging people to have a car on property - and a lot of reasons why they'd rather you just get on their buses.
(But you are right that the guard could have been far more polite and helpful)