As someone who worked for two not-for-profit organizations that relied on grass-roots support (and I’m familiar with a bunch more) I can tell you from personal experience that it is absolutely common for organizations to ask their supporters to send letters to the editor, state senator, Congressman, President or whomever AND to include sample letters.
In fact, there’s commercial software available that makes it possible for a part-time junior staffer with 30 minutes of training to implement an entire campaign.
I can’t say whether either the McCain or Obama campaigns encourage people to make stuff up in the letters, but the mechanics of a letter-writing campaign are, indeed, SOP.