You may have missed the bit on the Nebula site that says:
Any deliberate strategic coordination of nominations by sub-groups of voters for the purpose of placing one or more specific works on the final ballot (also known as slating) is prohibited. Reciprocal voting or vote-trading among individual voters (also known as logrolling) is also prohibited. Ballot manipulation of this nature goes against the spirit of the awards and diminishes the perceived honor bestowed to recipients of the awards.
More broadly, both those awards do expect that people will have read the entries and vote for the ones they think are the best of the year. There is also a strong tradition of not canvassing for particular works. Authors are encouraged to announce if they have eligible books out, but you finish that announcement by saying “please vote”, not “please vote for me.”
The voting rules may not say this, but it’s still part of how things are done. Seventy years of history provides a very strong basis for tradition. And the last time someone tried to slide around the unwritten (Hugo) rules, it provoked an absolute shitstorm.