Ask HN: Why do people use TM when talking about non-trademarked terms?

I have noticed with a fair amount of frequency people will commonly say things like "...It Just Works (TM)..." or, from a post today, "...real (TM) open source...". I see this here on HN, Reddit, and Stack Overflow.

What is the purpose of this? I assume it is similar to people starting sentences with "I mean..." or ending statements with question marks that are not questions (e.g., "I use this technology all the time?"). I do not understand what effect is trying to be achieved by this usage. Is it emphasis? If so, why not use underscores, which are used in Markdown to achieve the same?

Edit: added sites where I commonly see this idiom.