To add a little more detail: Let’s Encrypt is required to abide by the terms of various root programs.
Those root programs all require participating CAs to follow the CA/Browser Forum Baseline Requirements.
The Baseline Requirements, in turn, require certificates to be in accordance with RFC 5280.
RFC 5280 requires that subjectAltNames be in the “preferred name syntax” from RFC 1034.
RFC 1034’s “preferred name syntax” requires labels to contain only letters, digits, and hyphens.