@newhdanalyst
Users are prompted to register for MFA due to security defaults feature in Azure AD.
Microsoft is making security defaults available to everyone, because managing security can be difficult. Identity-related attacks like password spray, replay, and phishing are common in today's environment. More than 99.9% of these identity-related attacks are stopped by using multifactor authentication (MFA) and blocking legacy authentication. The goal is to ensure that all organizations have at least a basic level of security enabled at no extra cost.
Security defaults make it easier to help protect your organization from these identity-related attacks with preconfigured security settings:
One of the preconfigured setting in security defaults is "All users must register for MFA"
Users have 14 days to register for Azure AD Multi-Factor Authentication by using the Microsoft Authenticator app or any app supporting OATH TOTP. After the 14 days have passed, the user can't sign in until registration is completed. A user's 14-day period begins after their first successful interactive sign-in after enabling security defaults.
If you do not want this feature to be enabled in your tenant then you can disable it by following below steps,
- Sign in to the Azure portal as a security administrator, Conditional Access administrator, or global administrator.
- Browse to Azure Active Directory > Properties.
- Select Manage security defaults.
- Set the Enable security defaults toggle to No.
- Select Save

You can also refer below article for more information,
https://learn.microsoft.com/en-us/azure/active-directory/fundamentals/concept-fundamentals-security-defaults
Do let me know if you have any further questions.
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