I'm guessing Troy has been swamped by the influx of support emails from the release of 5.5 final. Greg Titus wrote the feature, but he is currently in charge of OmniPlan and so probably too busy getting version 1.0 out the door to be lurking in the OmniWeb forums.
I wouldn't normally try to answer here since I don't think I ever used that feature but here is my understanding based on vague recollections of the design of the feature 2(?) years ago:
If you view source on a page that you received via http, when you click the Store button, we attempt to store via http PUT. If that fails (the server rejects the attempt) we disable the button since obviously future attempts are not going to succeed (it starts out enabled because we can't really be sure whether or not it might succeed).
If view source on a page you received via ftp, then we attempt to save the page using ftp. You might need to view the page using a login and password in order for the save attempt to succeed (e.g.
ftp://mylogin:mypassword@myftp.mydomain.com/mypage.html)
This is my recollection but I could be completely wrong--however it does seem to match up fairly well with omniweb:/Help/sourceedit/index.html