AFAIK, the main legal issue is a trespass to chattels tort. The data collected is generally uncopyrightable if not reproduced in it's entirety without modifications. The relevant case is Feist v. Rural [0].
IANAL, but I think the best bet for staying technically legal is to use jurisdictional arbitrage and tit-for-tat to liberate the data. If someone scrapes a US server and are in the US and they generate enough load to deprive the owner of use, then they are technically liable for damages under trespass to chattels. If they instead trade scraping labor with people in other jurisdictions, then that other entity would be liable. There might be some other legal defense/attack that might be usable by the entity who has the data being liberated, but I reckon it would be tenuous at best.
IANAL, but I doubt that that is an issue in most jurisdiction. A website does not get to make their own law by simply putting up a note. Neither are the terms of use a binding contract between two parties because the scrapers are not the websites customers, and thus did not sign or agree to anything.
IANAL, but I think the best bet for staying technically legal is to use jurisdictional arbitrage and tit-for-tat to liberate the data. If someone scrapes a US server and are in the US and they generate enough load to deprive the owner of use, then they are technically liable for damages under trespass to chattels. If they instead trade scraping labor with people in other jurisdictions, then that other entity would be liable. There might be some other legal defense/attack that might be usable by the entity who has the data being liberated, but I reckon it would be tenuous at best.
[0] http://en.wikipedia.org/wiki/Feist_v._Rural