I see this is becoming a discussion on Twitter, but rather than tweeting about it and opening that can of worms on my feed, I thought I'd speak my peace here.
Putting aside whether or not sharing spoilers for attractions or a theme park land that is only in previews is moral, I wanted to address this strange legal question that has arisen.
The way I look at it, guests and passholders that were invited to the Super Nintendo World previews had to agree to not share photos or videos on social media before the land officially opens on February 4th. This information is posted on the registration site AND again on signage as you enter the land. Anyone that disobeys this rule is subject to having their annual pass revoked, or other measures that the park can take legally, including trespassing you from the park. The reaction may be severe, but considering these guests agreed to the rule prior to entering the preview, they are in fact breaking a rule set by the park by sharing photos online.
You know who didn't agree to this rule, legally speaking? Anyone that has not physically attended the preview in Universal Studios Japan. If you were to share a selfie from someone else's Instagram feed on one of your own channels, legally speaking, it would be up to the owner of that photograph to take legal action, (whether by DMCA or other regionally similar means,) to have it removed from your account.
Some may argue that art within photographs is also protected by copyright laws and could be asked to be removed by the artist, but that's when we start to get into a legal gray area, and may come down to the intent of the original photographer. (Taking photos while on vacation vs taking photos to sell in place of art, etc, but I digress.)
Should Universal Studios Japan have legal authority to police the sharing of photographs taken at a vacation destination, or should that right be held by the photographer?
Regardless, if you attended the preview, and you share photos before the grand opening, you can certainly be punished by the park, and I would highly suggest saving those posts until the 4th. But if you haven't attended yourself.... this is weeeird.