This very well may be something tied to programming by the WiForums folks, who provide the WAP mobile version of FT. FlyerTalk, IB, and VBB may not have any control over the WiForums code.

I do think there is some value in letting people know a message was posted from a handset instead of a full computer, as it lets people know to be more forgiving of typos and other mistakes. Emails from smartphones are often tagged with similar notes, which I find valuable even as a recipient, since I then know that the sender likely didn't have the time and screen real estate to do a full analysis of the situation or detail. If I were to receive a one-line email either saying something curt or, if a reply, not fully addressing my original message, I would assume the sender were being rude or not treating me with the time and respect that any human being deserves. The mobile device tag lets me know that such is not intentional but rather a function of the limiting interface available to the sender.

I also don't find them overly intrusive, given their small font, although there are a couple of downsides (such as that pointed out by the OP). If a more integrated way of notating the person was posting from a mobile were implemented in the vBulletin software (an icon or somesuch), that would be optimal, but that would understandibly not be on the top of the priority list for fixes to the forum software.