A lot of that is achieved by general vagueness in the wording of these rules. Paragraph 6.j of Yahoo's ToS (which includes Flickr) forbids "disobey[ing] any requirements, procedures, policies or regulations of networks connected to the Yahoo! Services, including using any device, software or routine to bypass our robot exclusion headers," which could conceivably include Web scrapers or other such applications. Most of the time, it would be hard for it to tell that those apps were in use, unless a great many people started using them, a lot of content from an individual user's account was being scraped or the service attempted to detect use of such tools and took steps to block them.
Users who ignore ToS provisions about third-party applications do so at their own risk. "Legally, you could be breaking a term of service or violating copyright laws," notes Hamilton. "Or, if a Web scraper is constantly scraping a site, they could impose performance issues or become the equivalent of a denial-of-service attack."
Reuse of your content. Some sites will have a ToS provision that allows whatever you post to your account to be redisplayed in other contexts. If you see this clause, don't panic, but do read it closely. This clause typically exists for the sake of allowing whatever you post to be shown in promotional material, rotated on the site's home page or just manipulated internally.
Google's ToS, for instance, has this in paragraph 11.1: "By submitting, posting or displaying the content you give Google a perpetual, irrevocable, worldwide, royalty-free, and non-exclusive license to reproduce, adapt, modify, translate, publish, publicly perform, publicly display and distribute any Content which you submit, post or display on or through, the Services. This license is for the sole purpose of enabling Google to display, distribute and promote the Services and may be revoked for certain Services as defined in the Additional Terms of those Services." Many other services retain a similar clause.
As-Is / As-Available. This is another catchall clause that, in effect, means the service has no particular obligation to provide continuous uptime, to protect your data's integrity or even to keep the service active. Note that As-Is clauses may be a bit buried and not broken out into their own section; search on the keywords "As-Is" or "warranty" to find them.