Maybe the barrier to posting a comment should be low if you don't have a hyperlink but if you include one then you have to jump through some hoops (captcha's, etc).
To be clear, we're talking about an automated process that is looking for forms to fill out. It's far easier and faster to simply scrape the HTML of a page and look for the <form> and <input> tags to figure out what to submit than to actually fire up something that resembles a real browser.
