Well, for the benefit of other readers besides the OP who has given up:
There is no mysterious “something” preventing VASSAL from running after you install it. We know exactly what it is–it’s the quarantine bit and Gatekeeper. The “download with Terminal” trick is simply to avoid this happening in the first place at the download stage, so you have clear sailing right from the initial launch after install with no warnings or preventions at all. If that sounds too complicated or intimidating, no worries! Don’t do it.
If you download the VASSAL installer the more conventional way, it’s flatly wrong to say there is “no other alternative” when the program will not initially run after installation. The aforementioned help topic link describes the procedure to overcome the security prevention and even annotates it with graphics along the way. In the worst of times, it’s about 8 additional mouse clicks–maybe even a couple fewer.
Is it exasperating and lamentable that a workaround is required to get going in MacOS world? Sure is! No argument from anyone here. But anyone can choose to do the workaround at the download stage (more technically complex) or the “after it failed to run the first time” stage.