A company like AsSeenOnTV.com isn't going to steal your idea. They are a web marketer like the rest of us. If you have a product, they will list it on their site and you'll pay them a commission on each sale they make. The rate varies, but its what you'd typically pay out to an affiliate network. In some cases they will buy the product wholesale from you, or times they will have you dropship.
The absurb share you're talking about, is when you license your idea over to a direct response marketing company. In this scenario you'll probably receive a 2-3% royalty on the gross sales. While that seems like they are ripping you off, they are the ones who are taking all of the risk.
They would pay for the product redesign if necessary, manufacturing, packaging, writing the TV spot, producing the TV spot, advertising the product, paying a call center, handling customer service and fulfillment and credit card processing, etc. Also, its important to realize that typically only 1 out of 15 or 20 products makes it. Most products are bombs and cost the marketer a ton of money.
In most cases handing the product over is the best choice for an inventor. What would you rather have? 3% of this year's breakout product being advertised nonstop on TV and that's in every Walmart and Target, or 100% of a pipedream that never gets off the ground.