Zillow, Trulia and other real estate websites need to make money so they ask real estate agents to pay monthly advertising fees to these websites. In return, these websites (including Trulia, Zillow, etc.) allow agents space on every listing they pay for. For Sale by Owners do appear at the bottom where it says "Listing Provided by xxx". On Zillow, the owner will appear under the agents faces. You posted your listing for free. These agents paid for your listing.
As for getting leads, buyers can contact you but they also can contact these agents. Agents need these leads to stay in business and these websites have default checkmarks next to the agents so buyers can easily contact them.
Bear in mind for Trulia, it's only designed for realtors to post listings - not owners. The best method doing For Sale by Owner is to pay a small fee to a flat fee brokerage or agent and have them list it in the MLS for you. You will be required to pay a buyer's broker but you will save 3% not listing it with a realtor. Going with a flat fee broker gives you 100% control over everything - including marketing.