Some websites are using -webkit-touch-callout within their CSS to prevent the callout menu being displayed on long touches. For example: If you visit thedailybeast.com on an iPhone, you are unable to long-tap the hyperlinks to copy link/open in new tab/add to reading list. The only workaround I can find for this particular website is to refresh into a desktop view. Maybe in the iOS Content Blocker there can be an option to override -webkit-touch-callout I am also seeing this behaviour on news.google.com, but after looking at loaded scripts and the CSS they generate, I am unable to find any reference to -webkit-touch-callout, although it may be obscured.
Ignore the last sentence about news.google.com - turns out that is using Javascript to block touch. I have raised bug 146901 in reference to this.
<rdar://problem/21797994>