Tiled services are a flattened version of your data where all layers are baked into a set of images. Once your tiles are generated, you lose the ability to control individual layer visibility when adding your service to a map as a tiled layer (the tradeoff is less control for better performance).
If your data behind the tiles is still on your server, you can add your service as a dynamic map service, which gives you the ability to control each layer's visibility. Dynamic services work with hideLayers while tiled services do not.
Hi Derek, thanks for the response. I was aware that I wouldn't be able to hide layers in the map view for a tiled service, but the legend is still dynamically rendered, when it is requested, right? I had some hope that one would still be able to hide some layers in the legend display! Should I enter this as a feature request on the ideas website?