I would venture to guess that AMD, Intel, Nvidia all engage in those types of activities to as much of an extend as their respective budgets allow. That is why using multiple sources is important, at the end of the day its close to impossible to know what is motivating any 1 individual reviewer.
But to think that Nvidia and Intel are engaging in this type of behavior while AMD is not, would be naive. Nvidia and intel just have larger budgets.
@that bf4 comment-- the benchmarks were probably run in single player, multi player is never going to run as well as a single player benchmark. And BF4 is an AMD game, nvidia would not being using it to highlight performance of their products.