*for sure, many people send/receive/store their private keys unencrypted, e.g. when generating paper wallets online
This usually happens locally on your browser except if you are creating your private keys through a malicious web page... You have to worry about the code the web page is serving you and not about your isp...
Generate paper wallets disconnected from the internet is the general consensus.....
To OP: Private keys should only be stored locally, on your own computer, encrypted. They should never be send over any network and Bitcoin does not send private keys when broadcasting a transaction.