This is a good real-world example and the response is an armchair programmer informing you that you are doing it wrong. The internet is rife with know-it-alls. "Just do X." Well, I cannot because I am contractually obligated to write the software as specified and not cowboy up and do whatever I like.

Maybe someone decided 100 characters was a reasonable cutoff and that field is not important enough to reject (read: increase bounce rate) on if someone manages to send too much.

Maybe the 100 characters is a short string generated from an unrestricted long string and cached on a separate server.