The papers often run stories sourced from various news wires or papers they have agreements with - Reuters, AFP, APP, NYT etc. and I doubt either the Western or Pakistani papers double check every fact in the story, though the editorial staff probably does make a decision on whether or not to run a particular story sourced from the News Wires.
In this particular case, for Pakistani editors at least, the claims made in the story from the original source were pretty credible, since Pakistan has suspected and alleged Indian involvement in terrorism in Pakistan for a long time, given past experiences with India supporting terrorists in East Pakistan and Baluchistan.
Some of the released cables (real ones) also have Karzai openly admitting to sheltering the most wanted Baluch terrorist leaders, to both US and UN officials, along with, as the Guardian pointed out, claims of continued Indian support for terrorists in Baluchistan sourced to British Intel (the UAE intelligence has also accused India of supporting the TTP).
Given the above, some of the Pakistani editors likely thought the report was credible, and ran it without verification, much like they and others do with reports sourced from the news wires and other Western papers.
There are other leaks that do imply that, and as far as 'alleging involvement in other nations' is concerned, the Western press hasn't exactly set a high bar in terms of providing credible evidence when vilifying Pakistan for 'supporting the Taliban and AQ'.
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