Hello geoi310 -
Currently the way the Shopping Cart Event works is an admin can set up a price value, and then specify the free product that will be added to the cart. As you have seen already, the product is simply just added to the cart automatically and does not let the customer choose whether they want it or not.
Due to this though there are a few options that you can do to work around this. If you wanted to give this free product out only when the customer buys X product, you can then add these free products as variant options. This way the customer can choose whether or not they want that free product or not.
If you wanted the cart to simply give a free gift though after $X, and let the customer choose it or not, you wouldn't be able to do this but the best method close to this is to use the shopping cart event as it works now.
What I can do though is make sure that this is a logged feature request though as I can see how this can be valuable.
-salestrainingseries - Currently MC is no longer offering custom programming but we do still offer design, and seo services for your sites. If you are interested in either of these checkout -
http://www.monsterwebdesign.com/http://www.monsterwebpromotion.com/Hope this can help,
Joe