Single character IDN domain names are punycode'd - should they?

Hi,

I was playing with IDNs, and found out that if a name is somehow legit [1], it won’t get punycode’d, e.g. λ1.com and δ1.com.

However, single character IDNs aren’t the case: λ.com, for example, is turned into xn--wxa.com, and δ.com to xn--pxa.com, etc.

I wonder if this is necessary.

1: https://wiki.mozilla.org/IDN_Display_Algorithm#Algorithm