This was, in fact, tested (to a limited extent) in court about a decade ago. See eBay v. Bidder's Edge, 100 F.Supp.2d 1058 (N.D. Cal. 2000).
Short story: Back in the days when there was actual competition in the online auction market (anyone remember Yahoo! Auctions?), Bidder's Edge was crawling eBay listings to index them for an auction search engine. (I worked for one of their competitors.) eBay sued on a trespass theory, and was granted a preliminary injunction because the judge held that eBay was likely to succeed on the merits of the claim.
Unfortunately, the trespass claim was never fully litigated; Bidder's Edge agreed to stop crawling after the PI was granted.
Short story: Back in the days when there was actual competition in the online auction market (anyone remember Yahoo! Auctions?), Bidder's Edge was crawling eBay listings to index them for an auction search engine. (I worked for one of their competitors.) eBay sued on a trespass theory, and was granted a preliminary injunction because the judge held that eBay was likely to succeed on the merits of the claim.
Unfortunately, the trespass claim was never fully litigated; Bidder's Edge agreed to stop crawling after the PI was granted.