Why? The ET forum has existed for so long with the rule that you could respond with a "LOL!" or a "+1" instead of typing out something like "I thought that was funny!" or "I agree with everything you said." Is it really necessary to force that?
Clutter. Some softwares, and probably this one, allow the "whozis likes this post" or "whozis thanks whatzis for this post". This eliminates extra posts which often serve only to lengthen the thread (and I'm NOT referring to Facebook "likes"). The advantage to this, particularly with regard to people maintaining journals, is that without it, and without comments, one can often get the feeling that he's just talking to himself, which can be discouraging.
I tried a dot and it wouldn't let me - said it wants 10 characters min. Then I tried a +1 - no go - same response.
Easiest way is just write what you want then add a few spaces, end it with a . Entering html code defeats the purpose of the OP issue