Monetizing your website with affiliate offers doesn't make it spam. Having no other purpose than to redirect a visitor to an offer - does. Especially if you use less than lilly white SEO techniques. Here's a nice way to put it:

Google believes that pure affiliate websites do not provide additional value for web users, especially if they are part of a program that distributes its content to several hundred affiliates. Because a search result could return multiple sites, all with the same content, they create a frustrating user experience.

And here's the rest of it: http://bit.ly/KWS1cz

>For example in the UK there are sites that offer money back on all purchases

I don't see the relevance.