Yes, the terms of service will generally be legally binding as a legally valid contract that you assume assent to if you use a website or service. Yes, from your description, if XMBC or whatever other service scrapes data from IMDB or Amazon in violation of their terms of service, they are more or less breaking a contract. See this writeup for a very similar resolution of a question before a court http://arstechnica.com/telecom/news/2010/01/browserwrapped-terms-of-use-still-enforceable-says-court.ars (users don't read TOS but court still finds them enforceable).
This would be more of an enforcement problem than anything else, however, if IMDB or Amazon tried to enforce its terms of service as any express or implied contract. You, as a user of XMBC or whatever other data scraping service, would likely be a bad target for them - really, if they wanted to enforce their TOS they would go after XMBC first with an injunction (basically a legal order to stop them from doing what they are doing) and potential damages. So if you are worried about liability, I wouldn't really worry too much about it as a user, you're a small fish to them most likely and not worth their time - better to go to the source. If you have a larger role working with that organization, you might want to rethink it. BTW these types of issues are why XMBC needs legal counsel, as is announced on their webpage (they are repped by the Software Freedom Law Center).
Hope this helped!