While Snaps can request the endowment:ethereum-provider permission to get access to the MetaMask API, it currently can't be used to call eth_requestAccounts. This prevents snaps from directly signing the user in using SIWE, or from any other calls to personal_sign and similar methods.
This repository shows an example of a snap requiring a ficticious API key to make some requests. The API key is obtained by doing a Sign-in with Ethereum. Since this requires calling personal_sign, which in turn requires access to an account using eth_requestAccounts, this signing currently has to happen in the Dapp.