Your posting keys can only be used to vote, author posts/comments, resteem and CLAIM rewards. It can not transfer funds or access any of the Steem, Steem Power or Steem Dollars your account has. The Active key is used for transferring funds. You can read in the steemit FAQ (link below) on how each key is used and why sharing only your posting key does not place your funds at risk. (This is the same reason why if you login to steemit with your posting key you are asked for your memo or master key to transfer any funds).
To directly quote the FAQ: https://steemit.com/faq.html#What_are_my_different_keys_for
Posting key - The posting key allows accounts to post, comment, edit, vote, resteem, and follow or mute other accounts. Most users should be logging into Steemit every day with the posting key. You are more likely to have your password or key compromised the more you use it so a limited posting key exists to restrict the damage that a compromised account key would cause.
[Fixed Memo to Active key]