Why are security against eavesdropping and security against CPA equivalent in the setting of public-key cryptography? Why are security against eavesdropping and security against CPA  equivalent
in the setting of public-key cryptography?
Under a chosen-plaintext attack, the adversary has the power to encrypt polynomially-many chosen plaintexts. In the symmetric world, since only the valid parties hold the encryption key, only the valid parties can "grant" the adversary access to an encryption oracle. So, it isn't assumed that an eavesdropper necessarily has access to an encryption oracle.

In the asymmetric world, however, the public key &mdash; the key used to encrypt data &mdash; is, well, public. So, an eavesdropper overhears the public key (or is simply assumed to know it, since it is public), and from there the eavesdropper may encrypt whatever messages he or she so desires. This is the same scenario as a chosen-plaintext attack.