(First of all, we aren't lawyers. The following is our layman's understanding of how American laws apply to us. This is definitely NOT legal advice to others.)
Despite ubiquitous user agreements and copyright notices everywhere you look, a recipe is NOT protected under copyright law. Only creative expression is protected by copyright law. Creative expression added to the recipe (eg: description or photos) may be copyrighted. One may also copyright a collection of recipes (ie: recipe book) because the selection of recipes, arrangement of recipes, layout, other text, etc. is a creative expression.
Interestingly, you cannot copyright a set of instructions, yet you can copyright the steps to a choreographed dance. What's the difference? The steps of the dance produce a work of art, and a recipe (among many other sorts of instructions) has, so far, not been explicitly declared as such.
Since there can be no copyright on a recipe, it is not legally necessary to give credit to the source. However, we do feel it is polite and will attempt to do so where we can. We also encourage others to do likewise. Please contact us if we have not given credit to the proper recipe source.
Here are some of the sources which led us to form our above understanding of how American law applies to recipes. Please contact us if we are wrong in our interpretation.