Well, the main alignments are certainly fenced off by a not-entirely-impenetrable 4ft fence! With gates, which, I believe, are unlocked outside of peak season...
It's not impossible, by any means, to get in amongst the stones, but I presume the desire to keep people out due to erosion is a warranted one, as it's a ridiculously popular destination!
There's plenty of big ol' rocks in the area that AREN'T fenced off, if people really feel the need to get in amongst 'em! And I mean PLENTY!!! The place is liftin' with 'em! ;)
G x