Fair Movie Reviews
US news media are not biased to favorably review movies made by their owners:
We find no evidence of bias in the reviews for 20th Century Fox movies in the News Corp. outlets, nor for the reviews of Warner Bros. movies in the Time Warner outlets. We can reject even small effects, such as biasing the review by one extra star (out of four) every 13 movies. We test for differential bias when the return to bias is plausibly higher, examine bias by media outlet and by journalist, as well as editorial bias. We also consider bias by omission: whether the media at conflict of interest are more likely to review highly-rated movies by affiliated studios. In none of these dimensions do we find systematic evidence of bias. Lastly, we document that conflict of interest within a movie aggregator does not lead to bias either. We conclude that media reputation in this competitive industry acts as a powerful disciplining force. (more)
A simpler explanation is that there is more profit in retaining good and reputable movie reviewers. If you forced them to shill, they'd go somewhere else.
And reveal that your publication shills for the firm's films.
Biased reviews don't need to be obviously biased (in fact it's not that difficult to write a convincing review with a positive bias). As it is movie reviews differ and the reviews are basically random with respect to corporate ties. If the reviews shifted more to coincide with corporate interests the public would not notice. Of course that would mean the average of the reviews would not matter so there would be no extra profit in it for any of the studios unless the shift caused at least one of the studios to get a positive bias from a bigger large news outlet or one that's more popular among their target audience.