Don't need to buy a ticket to walk around windmills of Kinderdijk

I have visited Kinderdijk three times so far to see dutch windmills.  Whenever I have visited there, I have purchased tickets to enter the area where windmills are located.  There are 18 windmills and only one windmill is still operative. 

When I visited there for the first time in 2012, I purchased tickets at the ticket office.  I did not experience anything strange there.  For second time, when I visited there with my brother-in-law two weeks ago, I purchased tickets online in advance in order to get discounts.  After arriving at the site, I showed the print-out tickets at the ticket office.  An old man at the ticket office asked me to go to a tourist center which was over a canal in order to get the tickets scanned.  It sounded really silly.  The ticket office did not have a scanner to read tickets that were purchased online.  I asked my brother-in-law's family and my family to stay in front of the ticket office and went to the tourist center to get the tickets scanned.  At the tourist center, an old lady asked to watch a video at the center.  In order to watch the video regarding the dutch windmills, I had to go to the ticket office to bring my families to the tourist center.  It was really troublesome.  I gave up watching the video and returned to the ticket office.  I told the old man that I had my tickets scanned and he just let us in the area where the dutch windmills were without ensuring the tickets were scanned.  It looked strange.  Anyway, we took some pictures and my kids rode bikes there. 

A week later, I went there again with my brother's family.  This time I did not purchase tickets online in advance in order not to commute between the ticket office and the tourist center.  In addition, my brother and his kid did not like to watch a documentary film.  Thus, I purchased tickets at the ticket office and entered the windmills area.  Again we took some pictures there.  This time, we got into a windmill that was operating with the tickets and had an opportunity to see inside of the windmill.  After seeing inside of the windmill, I thought that the ticket might be to get into the windmill, not enter the area of windmills.  When we returned to the ticket office to move to somewhere, I heard from other tourists that they did not need buy tickets to walk around in the area of windmills and the tickets were to get into the windmill that is still operative.  I felt like being betrayed.  The website for the windmills of Kinderdijk (www.kinderdijk.com) says that tourists should buy tickets in order to enter the area where 18 windmills are.