The stated reason is "in response to the difficult humanitarian conditions prevailing after the Israeli aggression on the Gaza Strip."
In other words, they don't want any videos or photos of happy Gazans to be spread around because they are vested in showing the world nothing but misery and destruction. Gazans having normal lives means less international funding.
Gazans are still allowed to have celebrations in private.
In a honor/shame society, "optics" is inherently more important than reality. How things appear is what generates honor or shame, not how they really are. Putting on a show to the world is as natural as breathing. That's one reason why public self-criticism is so rare in Arab society - shame must be avoided at all costs.
As we have seen from "honor killings," shame is often worse than death.
This means that literally everything that Western observers see in Gaza and the Arab world is being presented through a distorted lens, where facts are hidden or fabricated or spun as needed. The competing desire for honesty that Westerners have in their culture is subsumed to the requirements of honor and avoiding shame.
This is a small example of this phenomenon, but it is terrifically important to understand. While today's Jews are anxious to admit their shortcomings to the world and try to explain them, Arabs will simply hide them.
Without understanding this, one cannot report reliably from the Arab world.