Is it rude not to attend courses which have no attendance requirements?
I think the question is not "Is it rude?" but "Do lecturers find it ok?" as the answer to the latter is not universal but varies a lot. 

Here is what I think about students skipping classes: Personally, I also do not have any attendance requirements (actually, where I am it is currently forbidden to have such requirements) for my classes and there are students who skip some or most classes. Since I do not have such requirement, I am not mad at them, but I note that they miss opportunities to get into a dialogue about the contents which help them learning the material deeper. I do get mad when these students come to office hours asking for clarifications of stuff I treated in the lecture since this basically amounts to unnecessarily teaching the stuff double. When I note that students skip many classes I get thinking "Hmm, let's see how they manage to grasp all the material…". When it comes to exams and I note that the students missed some point which has been stressed in the lectures over and over, I feel like the students really missed a crucial point (even if it is just one) just by skipping the lectures and I do not give any benefit of doubt. It's like "it was obvious that I considered this to be a central point, and you did not get it - sorry for you." However, some bright students that skipped classes managed to impress me during an exam and scored great grades.

As for not greeting students: If you haven't shown up in classes that often, your face may probably not be that familiar to the lecturer… Here in Germany we often have hundreds of students every semester so it is impossible to know the all students who are enrolled but did not show up.

As for the question What can I do to turn it around?: Show that you learned a lot by being great at exams and show that you deeply grasped the material. Do not go to office hours with questions that have likely been answered during the lectures.