Your runs are tied to your shoes. Therefore, you can't delete old shoes.
But why would you want to delete them? If you've got runs in them - keeping that information in tact only makes sense. If you add shoes that you've never registered with a run, then you should be able to delete those. Just click on the "Running Shoes" menu option and then click the "X" next to the shoes you'd like to delete.
I'll move this post to the Suggestions area and will also add the information to the SiteWiki in the Running Log area.
Thanks