Bought a camera in Hong Kong and had to pay tax at Shenzhen border?

Hi,
I'm from the UK and I bought a DSLR camera in Hong Kong. When crossing the border at Lowu in Shenzhen the customs officers stopped me and told me that I had to pay customs duty, even though my camera is for my own personal use and I would be taking it back out again!
I was charged 700RMB and I had no china currency so I had to get it changed too.
I didnt quite understan why they taxed me and it is very strange since I'm a tourist. Please advise.
Thanks.