The reason is because Route::resource
creates several (RESTful) route handlers for you in the background for the controller you specify:
http://laravel.com/docs/controllers#resource-controllers
Take a look at the table called: Actions Handled By Resource Controller
You can see that Laravel will already handle routes for you that take parameter that you can use to implement browsing.
I don't think that the intended usage for Route::resource
is to be parametrized like you are trying to.
Of course you can always implement additional routes if those do not match your needs.