Last night I first banned Shaco, then Ahri and Kass. I purposely did not ban Rammus or Ryze or Poppy because I knew I'd have first pick(I'm not very good at Kass yet). Opponent banned Yorick Heim (don't get that one) and Poppy. I picked Rammus.
Ok, so that happens. What doesn't make sense comes next. They proceed to have 3 strong physical damage champs which weren't particularly tanky and had very little CC, and not very strong magic.
Getting Rammus doesn't mean auto win, but if you don't like Rammus ban him first or get kiters/CC/magic damage. I think this applies to each champ. Poppy can be countered with people who hit harder at lower health (Pantheon for instance).
Each of the top picks/first bans can be countered, but the majority of people do not think they are fun to build against. I've seen Teemo with sweeper invalidate both ap and ad shacos. The shrooms cleverly placed prevent back doors and the Shaco becomes like an Eve who either team fights or makes the match a 4v5(hurting their own team).
I ban Shaco for instance, because unless I know all 5 people on my team I can't expect that the logical person will go stop Shaco when it counts instead of running around stupidly in the wrong place.
I ban Ahri because I personally hate what she does to team fights. I know people who ban Blitz because they hate what he does to team fights. When I know everyone on my team knows how to dodge Ahri and Blitz's skill shots... we don't ban them (and in fact try to get them at champ select).
TLDR: Rammus ban is about personal and team play style, and the majority of people do not like fighting a champ who has so much CC, speed, and armor when the current popular meta is tanky physical dps.