Thou shall not pass...
When they first opened the southern pedestrian walkway that provided access to the observation deck, there was no gate or barrier. They quickly added a portable steel grate that spanned the width of the walkway and attached it whenever they wanted to block access to the observation deck. I recall that grate being used to close the deck 24/7 for many days following the skateboarding incident. We now have version 2.0, a formidable permanent solution that is intended to keep everyone out of the observation deck area after hours. It also matches the robust security door to the control room / bascule pier in its aesthetics and "shininess".
Could be worse... we could live in a country where they put you on a list and interrogate you for taking pictures of bridges!