This is the result of the tax laws. A Roth IRA custodian has no way to know your basis in Roth IRA contributions because the law treats all of your Roth IRA accounts as a single account for tax purposes, and any given Roth IRA custodian has no way to know of any Roth IRA activity you have in Roth IRAs at a different custodian. That makes it entirely your responsibility to track Roth IRA contribution and conversion basis.
