At least in the tierless format, the Masters are playing within the rules designated by WGT. Though it's probably not the wisest format in terms of equal competition, it's not really fair that Masters be called on an 'ethics infraction' for playing within the rules set forth.
However, the 'loophole' is an unintended side effect of tier promotion, so I think we're dealing with two separate issues here.
The loophole should be closed and tierless tournaments should be considered a failed experiment.