It's "taboo" because some people don't know how to respond to others that do things they themselves don't partake in or enjoy, and then mock those that do. Been RP'ing since FFXI and aside from the odd mt I've never seen RP "forced" on people. Also never seen someone RP character traits in dungeons that hindered their role in anyway (one day I will see the Ice Mage unicorn T_T)

Just look at any fandom, subgroup, or hobby that has any sizable number of people. There will always be people who consider them "odd" or "not normal".