Because Mayonnaise is and has been defined as:
http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRSearch.cfm?fr=169.140
Salad Dressing:
http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/cfrsearch.cfm?fr=169.150
It’s been a long time since I was in the business but I believe that a product like this would be called a sandwich spread, which is somewhat ill defined.
This is true, there are such rules, some sillier than others (which is also why there have been brouhahas over Swiss cheese whose holes are not large enough).
Still. If nobody has defined the word Mayo, then it’s open for use in a trademark on a “sandwich spread.” (I bet it would make fair salads, though.) I’d expect Kraft with their “Mayo” mayonnaise would be complaining about trademark issues.