I bet they wouldn't feel that way if they had managed to be among the lucky few that got to ride for a soft opening.
Seriously, though, if the CM's are only allowed to come and ride it when the MK is closed, then fewer of them might take advantage of the opportunity. In addition to the other reasons that
@Tom mentioned for why these tests being done during regular hours is an important factor, something else to consider is that, if the only time Disney offered CM previews was during oddball hours when no one wants to be there, the CM's who need the practice of running the ride and the testing that needs to be done won't be happening under conditions that are close to what the norm will be. They need a steady flow of people to get the CM's used to the timing of loading and unloading, among other things.
In fact, I wouldn't be surprised if the soft openings for the public were being dictated, at least in part, by whether or not there is a decent sized break in the number of CM's in line, or even because they want to add to the number waiting in the queue for timing purposes... after all, people are going to want to know how long the wait is once the ride opens, and the CM's working have to know how long the wait is from different points in the queue in order to make that happen. None of this would work out if Disney was only allowing previews to take place during the graveyard shift.
Besides, to those of you who are screaming that Disney is acting poorly by allowing the ride to run for CM's only during the day, do you really want the CM's who are operating other rides during their shift the next day to be running the rides on little to no sleep because Disney encouraged them to stay up all night to ride the 7DMT? It's one thing for them to choose to stay up all night on their own accord, but Disney doing anything that would encourage them to make decisions that could impact guest and CM safety sounds like a bad idea.