It's manipulation of the glass door rating because it alters the organic state of reviews. You're not getting a baseline of reviewers that were motivated enough by their experience to go out and leave a rating. If every company were doing this same thing, then for the candidate looking for insight it would still be reasonably easy to get a fair comparison. With some companies manipulating the results and others not, it's not a fair comparison.
To claim that it was not manipulation, it seems to me that you'd have to argue that the reviews that were created under this encouragement would have all happened anyway, without the encouragement, which I find to be a dubious claim.
I wonder, if a product manufacturer were encouraging their employees (that were themselves users of said product) to go on Amazon and review the product, would you consider that manipulation of the Amazon reviews?