Why does the GTM adapter rewrite top-level event schema in trackEvent?

I'm referencing [this block here](https://github.com/adopted-ember-addons/ember-metrics/blob/master/addon/metrics-adapters/google-tag-manager.js#L52-L55). All the top-level keys are rewritten with the prefix `event`. This breaks Enhanced Ecommerce tracking as it looks for the `ecommerce` key specifically.

Obviously I can easily override the adapter to not do this, but I'm wondering if there's even a good reason to do it in the first place. It seems like, all things being equal, the adapter should just write the schema I told it I wanted. As far as I can tell the code has been there since the introduction of the adapter and there was no explanation for it in the relevant PR.