Whether a Service is available depends on two things: the type of service and whether the application can accept services (e.g. certain Carbon applications).
In this case, it is the type of service. The Grab service lets you paste a screenshot into a file. As you are viewing a web page in Safari, you cannot paste an image (just as you cannot paste text onto a web page, unless you are in a form).
Try out the service in TextEdit, and you will see what I mean.
On the other hand, try the service "Make New Sticky Note" in Safari. As this service takes the input (such as selected text) in the current application, it will work in Safari.