The R side chains are not referred to as radicals. R- is common in organic chemistry and refers to a part of a molecule not shown for some reason. Generally because it’s not the focus in the specific figure. Generally you’ll have a R = (whatever combination of atoms) defined below.
Radicals are a specific atom with an unpaired valence electron. Sometimes also called “free radicals”.
Edit, here’s an example of a non-amino acid use of R: