Google now has updated their review guidelines to prohibit and disallow negative reviews from current or former employees.
The new section under "conflict of interest" reads:
Maps user contributed content is most valuable when it is honest and unbiased. The following practices are not allowed "posting negative content about a current or former employment experience."
Here is a screen shot:
Joy Hawkins goes into more details about the change on Search Engine Land.
I guess this is a good thing, you don't want disgruntled employees airing your dirty laundry, right?
Forum discussion at Twitter.