NaCl allows decryption with the same public key as for encryption
Alice and Bob's keys are used to compute a shared key.

Both parties compute the same key, which is used both encryption and decryption.

This is a traditional, hybrid encryption system.

Using only asymmetric cryptography for encryption would be painfully slow.

As a mitigation, the sender can create an ephemeral key pair, deleted right after the message has been encrypted. See ECIES or libsodium's crypto_box_seal() construction.