That's because everything between the shorelines, including bridges, can be OB depending on how the OB area is described or delineated.
For example, if OB is described as "in the river/water" then it's easy to say that a bridge over the water would be in-bounds as it was not designated as part of the OB. But if, rather than using the water line, the river is marked with paint or rope and OB is defined as within the marked area, then I'd argue that a bridge within the roped/painted area is a part of the OB area.
There should be no assumption either way, IMO. That's why it's so important for TDs and course designers to be absolutely clear about what is and isn't OB rather than give vague or generalized descriptions. And players, if they have any doubts, should ask for clarification rather than assume something based on a past experience, likely on a different course with different people in charge of making rulings.