In our Idioms page, we have now made it easier for users to submit via an online form. In the idioms database, we take a broad view of what an idiom is.
As our site is for people who are learning English as a second language, we include things like prepositional phrases that might cause confusion to a learner, so, for instance, on the wagon is included. We also include sayings and proverbs. Any commonly understood phrase whose meaning cannot be deduced from consulting the individual words in a dictionary is regarded as fit for inclusion.
It's logical to consider idioms in this way. I agree.