[mcherm]

I can think of no reason why there would be any such restriction.

Suppose Facebook is getting paid by Bing, and won't offer crawling to those that aren't paying it? Suppose Facebook considers Baidu's crawler to be evil and chooses to prohibit it for that reason? Suppose Facebook just kind of likes the guys at Bing and decides to allow them special access? If you agree in the first place that Facebook should have the right to put ANY sort of restrictions on who can crawl their side, then why should ANY of these be prohibited? This is not a "common carrier" kind of situation.