IANAPMP - but there may be a legitimate reason. Caremark is what's called a Pharmacy Benefits Management company (or PBM); they work with health plan sponsors to "provide the most cost-efficient and comprehensive prescription benefit plans." Presumably, one of the ways a PBM tries to be "cost-efficient" is to look for generic equivalents and get subscribers to switch to them when appropriate.

CVS and Caremark merged last year. If your wife's health plan uses Caremark as its PBM, such a letter would make a great deal of sense, despite the overall CVS-less-ness of northern Maine.