It doesn't matter what Disney did, some would have complained about it. If they left the walls up while CM training was underway then some would have complained 'what are they hiding?'. They took the walls down so everyone can see it and now they are complaining that they can't ride. There are some in the community that will find something negative in everything Disney does.
Every new attraction has a readiness schedule that they have to go through once it's handed over from construction to operations. That schedule has a whole set of mandatory tasks the team need to complete leading up to the launch date. It's reasonable to assume that schedule is what dictated the 5/28 opening date. It would be irresponsible on Disney's part to open the attraction one day before they have completed all of those readiness activities and signed off on all of the readiness milestones in that plan. Sometimes that process goes very smoothly and it creates the opportunity for them to consider brief 'soft opening's' and sometimes it goes right on schedule.
Absolutely no guest is 'entitled' to any access to this attraction until the schedule and announced opening date of 5/28. Absolutely no guest has any right of any kind to treat a cast member disrespectfully because you want to ride an attraction that is not open yet. There has been no surprise about the opening date for weeks now and if you have expectations that you would get on before 5/28 those are 100% your issues, not Disney's nor the CM's at the entrance.