Why are 712 delegation signatures disallowed in MM Core?

I'm trying to use the MM Delegation Toolkit to implement sponsored txs in my dApp which requires users to sign a delegation payloads. After several hours implementing MM Delegation Toolkit everything was going well until I finally requested a signed delegation and I get the error: ```External signature requests cannot sign delegations for internal accounts.```

I traced this error to the core package here:

https://github.com/MetaMask/core/blob/main/packages/signature-controller/src/utils/validation.ts#L249

I'm just trying to understand the usecase of the Delegation Framework if developers can't leverage its features from a webapp connection to the user's MM.

Start with packages/signature-controller/src/utils/validation.ts at the linked validation check, then read the surrounding signature-controller logic and the MM Delegation Toolkit usage described in the issue. Determine whether rejecting external signature requests for internal accounts is intentional; done means the supported use case and rationale are clarified, or the required behavior change is scoped.

Written by the indexing model from the issue text.