No one is asking that anyone give out or post their private key.  That should be made very clear!

The requirement for sending BTC from a BTC wallet that you control the private key to is important.  For one, if you don't control it then someone else does.  If you send BTC from an exchange, then the exchange you sent from owns your Peerplays...because they have the private key to the wallet that you were using.  And often times, the address is just one of many inside of the wallet that they are using.  This means that there is no way to prove that you or someone else sent the BTC for Peerplays.

For two, and I am assuming here, but in order to claim the Peerplays bought in the crowdsale, you will just input your private key into the Peerplays Blockchain when it launches at the end of May. 

Your private key is your proof and the method for claiming. There is no need to ever post it or give it to anyone! But you need it.  And you need to know how to find it and use it.  Per the advice I gave earlier, if you have questions, then utilize the support team of whatever wallet used/that was recommended. They understand the importance as well and are great resources.