From the The Photographers Rights:
QUOTE
Property owners may legally prohibit photography on their premises
but have no right to prohibit others from photographing their property
from other locations....In any case, when a property owner tells you not to take photographs while on the premises, you are legally obligated to honor the request.
The Seattle Art Museum owns the Sculpture Park property - therefore they can do as they please - though they cannot stop photographers taking photos of the sculpture FROM public spaces adjoining the park.
If they're trying to block photography from the public right of way as well (and that was in no way mentioned as actually happening in the blog posting, nor could I find ANY reference to the photography prohibition on this particular sculpture online at all) then the SAM doesn't know the law, and I somehow doubt that that's the case. Tempest in a teapot on this guy's blog?