Yup, it's gambling, but so is spending any diamonds on events, it's just less obvious.
Regarding the legality of loot mechanics in gaming: at some point, the hand-holding should stop IMO.
Sure, gambling + kids = bad, but why does your kid have access to your MasterCard?
If not a kid, then paying money to buy diamonds for something you can get for free is your first choice with choosing to spend them on anything being a second choice.
Where does personal responsibility start?
That being said, I'm a very big supporter of "If I do this, I will get that" game design with the element of chance being minimal.
If the odds aren't reasonable I vote with my wallet.