Those translators (an LPFM is different) usually require an AM in order to be on the air. So even if they wanted to turn the AM off, they could not without losing the FM authorization.We have LPs littering our dial, including one that is literally a simulcast of a local AM station running the same Catholic programming.
(Some translators do not require a local AM...such as non-commercial operators like K-Love that use them to cover many areas of the country)