I do feel for you and I do think a lot of companies have extremely poor HR functions, failing management structures and managers that can't communicate with HR. It is so rude when companies do this and they should have some decent manners and either tell you whether or not you've been successful, or inform you if and why there appears to be a delay in coming to a decision.
So, yes it is rude, but sadly in my experience, it appears to be normal.
I was recently in a very similar position. Like you, I had a final interview for a senior position. Previously I had passed the application stage, an on-line assessment of my management style, a first round interview and then I had been given a task to do. I was asked to write a "cabinet paper on reforming public services", I put about 20 hours into this task. I had my final interview and was told I would be told the outcome by the end of the following week. I allowed two weeks to go by, and heard nothing.
I contacted the company's HR department and was told I'd receive an update soon, but the original person was no longer dealing with it. I waited a week. I then contacted him back and asked for details of the person who was now dealing with it. I emailed this person and waited another week. Heard nothing. I emailed them again to get an apologetic reply stating that the manager that had interviewed me wasn't responding to them. Another week went by, and I chased again to be told they were trying to get a response from the second manager who had interviewed me. Another week passed, and I chased again and was told I would receive a response by the end of the week. I emailed them at the beginning of the following week to be told I wasn't successful as it had been decided that they didn't think my skills were transferable from the industry I had worked in. I was livid
. I felt like responding, with words to the effect of, "what, you mean you need managers who can't make decisions, are unable to communicate with HR, cannot provide constructive feedback and waste a lot of time?"
My advice would be to contact the company in writing or by email, and politely ask if they could communicate whether a decision has been made with regard to the position. It may be that they haven't made a decision (but it is best practise to communicate that there is a delay), it could be that you are second choice and they are waiting to see whether their first choice candidate accepts or rejects (although again it would be best practise to communicate this to you), or it could be that they have a shoddy managment structure and a complete failure in communication, in which case, do you really want to work for them?
Whatever happens, I think you have the right to know the outcome of a second round interview, and receive constructive feedback if you have not been successful. You've put time and effort into the interview process; they should thank you and reciprocate.
I really do hope there has simply been a delay in communicating to you or making a decision. If there has been, best of luck. 
PS Sorry if I sound so overly negative - my personal experiences job hunting of late haven't been good!