There's no reason they should -- its either poor application planning or lazy development. Javascript-heavy web apps that use hash fragments or pushState can (

There's no reason they should -- its either poor application planning or lazy development. Javascript-heavy web apps that use hash fragments or pushState can (and should) use fallbacks to allow navigation without javascript enabled (especially important for search engines). At this stage of web app development, many just don't, but expect this to change as the JS framework world matures (hopefully)!