Multisite sub-folder name restrictions – can they be removed

I have a multisite, sub-directory based WordPress installation and on the “Add New Site” screen, it states that only lowercase letters and numbers are allowed for the Site Address field, i.e., the sub-folder that the site will be mapped to. I would like to use dashes and underscores (in addition to lowercase letters and numbers).

Questions:

1. What is the purpose of this restriction?
2. How could I disable this? Is there a plugin I could use or failing that, could someone suggest the appropriate filter or action that I could use in a plugin to try to disable this restriction?

Thank you for your time.