My guess is they like the work Sauce and their modders do very much.
It saves Zwift money and manhours, since it is all Sauce and their modderd who do the work. They keep it running, updating and improving without that costing Zwift anything at all.
as a bonus they cant be held responsible if anything goes wrong since its a 3rd party mod.
And bonus 2, every sauce user has to have a 2nd zwift acount. Even if that is not payed for and never used. They can still add it to the registered users number.
If the end up is to create register users, they can just use a script and manufacturer users. 900 trillion users overnight.
I also don’t think Zwift is trying to help modders. If they were they would provide the api tools for them. Imagine if Zwift actually provided tools for modders, you can actually have roads and worlds created by modders. How about a Star War base world? Or someone recreates cycling from Hawaii’s Mauna Loa. No Zwift can careless. They can’t even provide organizers with tool to succeed or a discard old bike button for the garage.
Hell you wouldnt even need a 2nd account as a work around if they wanted to help modders like sauce.