Properties of each of the structures clearly highlight the semantics of the work of this structure. If some structure works with energy only, adding a `store` property containing the sole `energy` property and incapable of containing any other properties would confuse the API user. This structure just needs energy to work, it has no “storage” to store anything, it’s just an energy device. So this API expresses structure functions in a more consistent and clear way. As for the labs, they can’t contain more than one type of minerals, and this also should be evident from the API.
You just shouldn’t interpret different structures in the same way as you work with their objects. Especially because they should be handled differently from the gameplay viewpoint.