The pressure to leave a tip can probably be connected to the cultures that we all come from. In the American/Canadian context, it's considered rude to not leave a tip at a restaurant or when receiving a service, but in Japan for example, tipping is not expected at all (and actually leaving a tip might be considered rude!). When I was travelling in Europe, in some of the countries that I went to, tipping a small amount was expected, but not if the service was poor and not to the same extent that it is in the United States.
I personally don't find it rude for people to say that tips are appreciated, but I would find it rude if they get mad at people who don't choose to leave a tip at all. After all, they did say tips appreciated. I sometimes also find it helpful if people leave general lists about what kinds of tips they like because I worry less about giving them something they might not want.