Of course it is only an assumption, but it is one I have come to after years of watching her and other young royals and wealthy rich kids.
What I believe is that Prince Albert knows that all of his children will inherit his wealth. I think it would be irresponsible to have an adult child suddenly inherit millions and have no idea how to manage it. A trust would be managed by representatives with an allowance coming from it. Usually a child comes into control of the $$$/investments at a certain age. A nice house in California would be a nice investment.
All my assumption, I should not have stated it as fact. I have spent a lot of time on this thread trying to figure out how she had these designer gowns and trips. I finally came to my own conclusion that, like many others, she was a trust fund kid.