I also do not see the reason for not allowing someone abstaining.
There are colleagues that vote on names that do not exist just because they do not want to vote, which is the same as abstaining.
How are you going to force a colleague to cast a 'real' vote? It is just not possible.
Maybe the idea is to avoid someone pressuring peers not to vote?
There might be extreme situations when that could occur and maybe this rule is for those exceptions?