Why is everyone assuming at face value that the threats are actually coming from gamblers, and not from the Israeli government trying to downplay the domestic impact of the war?
Because that's a silly idea. When missiles are flying over people's heads and everyone is running into bomb shelters, Israel would get approximately no benefit from nitpicking a single story, certainly not enough to justify a harassment campaign involving lots of people with cover stories. On the other hand, gamblers have a very concrete motivation and a long history of doing unhinged things to get it. There's no reason to look for depth.
Probably because it's 100% par for the course for online betting, unless you think it's Israel sending death threats to French tennis players for blowing people's prop bets.
I've seen crazier conspiracy theories...
Because it's in the Israeli government's interest to make the threat from Iran look bigger, rather than smaller?
You could reasonably argue that both are in their interest, and which one they choose to push is down to a decision. The war already has popular support in Israel (last time I checked, the quoted number was 86% in support) so the government wouldn't gain all that much from extra fearmongering, especially with the messaging coming from the Iranian government.