It doesn't that's why they have:
> Severability
> In the event that any provision of these Terms of Service is held to be invalid or unenforceable, the remaining provisions of these Terms of Service will remain in full force and effect.
In Australia, Europe and the UK, there are laws that make "unfair" clauses unenforceable... where unfair includes all kinds of things you see in every consumer level contract like no liability for death/injury or avoiding delivery with no notice or compensation.
In the UK the legislation is 'the unfair contract terms act 1977'. http://www.legislation.gov.uk/ukpga/1977/50
Also, the case law suggests that if you are providing a professional service your appointment should make it clear that you will take 'reasonable skill and care' in carrying out the services otherwise you will be judged as to whether the service you have provided is 'fit for purpose' which is a much harder test to pass. I dont know if this test would apply to SAAS T&C's and IANAL by the way. Edit: Clarity & Bad grammar.
In Australia, Europe and the UK, there are laws that make "unfair" clauses unenforceable... where unfair includes all kinds of things you see in every consumer level contract like no liability for death/injury or avoiding delivery with no notice or compensation.
For US examples, see "The Puzzling Persistence of Unenforceable Contract Terms" http://moritzlaw.osu.edu/lawjournal/issues/volume70/number5/...