Actually I think that's a common salesman trick ? Put the product in the hand of the customer, so that they feel they already have it, then they have to pay.
Its that too, but that aspect of it isnt gambling. It's the part where buying the existing loot box gives you access to a second, yet unknown, gambling outcome.
I can see how this can work in the US, but courts in Belgium (and Europe in general) do not stop at technicalities and they can absolutely consider it gambling without having to define the exact boundaries of gambling.