You have a problem and you need to address it.
You have three options :
Try to ignore it and go on as before ( and maybe sulk or get bitter as options ). How much is the free access you get worth to you ? Enough to swallow what you regard as offensive ?
Talk to the centre management and explain ( politely ! ) your issue. See where that goes. Remember that they won't see it your way and don't expect them to immediately agree with you ( or ever agree with you ).
Simply email them and tell them that you won;t be doing any free work again because of this. This has the disadvantage that you lock yourself into a position you can't negotiate easily from.