Question:

Why CRM Address Line 1 on the Address Object Page Layout cannot be set to Read-Only?

Answer:

Address Line 1 is the Name field of the Address Object. The Name field is not an incremental naming convention field and must be filled-in. The only way to make this field Read-Only is to make the Address Object Read-Only per the User's Security Profile.

Related Documentation:

CRM Help Documentation: N/A