isn't it then a bit dangerous to allow the "offended" to dictate what is allowed and what is not?
I didn't say that they should, and I'm not sure that they're mutually exclusive.
In this case, I think it's perfectly possible to acknowledge the offence caused (which, while I don't share it, I find reasonable) but still say that, as a significant, historic piece of art, it stays, the owners make clear that by keeping it they're not condoning or agreeing with any of the connotations that can be read into it.
It can, as has been said, be a positive learning resource, a jumping off point for a "hey kiddies, people didn't always think about people with different coloured skin the way we do today, and what do we think about that?" conversation, and we don't forget about our history by hiding away the uncomfortable bits.
I'm happy with people being allowed to be offensive as they choose within the bounds of round about where we are legally, so inciting hatred or violence is about where I think someone needs to step and in say "the line is here". I was never comfortable with that thing that happens a lot in student unions where they "won't give a platform" to, say the BNP, or hardline islamists. Remember the furore about Nick Griffin on Question Time? Have you ever heard Nick Griffin speak? The best way to ensure that people like that have no credibility isn't to hide them , to martyr them to "political correctness" or whatever, it's to give them the full glare of the publicity they seek, and expose them.
So, yep, I totally agree that letting the offended dictate what is allowed is generally not the answer, but I don't think that's a necessary consequence of accepting that something is offensive (to some people).