The bitstamp address "to trust" is
rvYAfWj5gh67oV6fW32ZzP3Aw4Eubs59B, however when you withdraw money, it you get it from
rrpNnNLKrartuEqfJGpqyDwPj1AFPg9vn1 instead. Why?
With weexchange I got the money from the same address I trusted.
They use hot and cold wallets for security reasons. The address you trust can create an infinite number of IOUs and so its key must be carefully secured. The address they transferred to you from must be able to perform transactions easily and quickly and so cannot be fully secured.