phranque - 11:25 am on Aug 21, 2012 (gmt 0)
often these are spambots that probe your site by submitting forms with unique strings for the form inputs to see if the content gets published.
later there may be additional probes to see if urls get published as anchor links and whether or not they are nofollow.
eventually the goal is to publish spammy links on vulnerable UGC sites or in comment spam.
the spambot doesn't "know" it's a subscription form.