That's because Chrome's permission model isn't very granular. If you're doing something that needs to be able to interact with any website (like that tab suspender thing), you get that scary warning, regardless of whether or not it's actually reading or changing data on all websites, because the permission indicates that it can, and there's not a less granular option available to declare.

The other option for extensions is to specify certain sites that they work with (this is the "read and change all data on 'x.com'" permission prompt). That only works for site-specific extensions (like Reddit Enhancement Suite, or Camel Camel Camel).

you can get the source and check it out yourself: https://github.com/deanoemcke/thegreatsuspender

Blame Google for a lack of granularity in their permissions.