Re: Why do they care?
First up, you don't really need a published legitimate interface to ride on the back of someone else's service, just some web scraping (aka screen scraping). One way to fight back would be an eBay-esque term of use that expressly forbids that kind of processing (a trick also employed by others, like XE.com's currency conversion site).
Of course you can always go ahead anyway and bounce through the Tor network to avoid detection (not that I would do such a thing) but it does make it a little more difficult for a casual scraper and of course in cases like lastminute.com they have to admit their abuse of your system in order to provide their service to their customers, so there's no real way to hide in the end.
In answer to the question I claimed to be replying to, the reason's pretty obvious and not very respectable. Ryanair want you to go to their site to buy tickets, which means you aren't going to be able to conveniently compare what they offer with others. The problem is not so much that lastminute.com sell Ryanair tickets, but that they sell non-Ryanair tickets.
Same sort of issue that some insurance or financial companies have with price comparison websites: seems like free advertising until you realise you're permanently the seventh-cheapest option and nobody uses you for anything other than a justification for buying your competitors' services.