We have to remember that if a cryptocurrency has a small market cap, an MPA might "steal" a lot of its volume and make the market cap even smaller. MPA is not the real thing so in a way it is like a competitor. If a big whale decides to change his cryptocoins to MPA-coins, he has to sell the real coins on the market, which will cause price to decline. If everybody does this, the underlying currency will become worthless, and so becomes the MPA too, because it tracks the value of original asset.
On the other hand, UIA does not cause prices to collapse. If you have UIA-coin on the Bitshares blockchain, you have also equal amount of the original currency in the gateway's wallet.