The question is, why should a company in Amazons situation be unable to turn off that service? As in, your question assumes that Amazon turned off Parler out of nowhere for no reason other then "we dont like them". It assumes they did not had documented reasons, documented attempts to convince Parler to comply to TOS etc.
Personally I found Amazons response to Parler convincing: https://cdn.arstechnica.net/wp-content/uploads/2021/01/gov.u...
The question is, why should a company in Amazons situation be unable to turn off that service? As in, your question assumes that Amazon turned off Parler out of nowhere for no reason other then "we dont like them". It assumes they did not had documented reasons, documented attempts to convince Parler to comply to TOS etc.
Otherwise said, contract.