Your score will be determined by the number of questions you answer correctly. Questions you answer incorrectly or for which you mark no answer or more than one answer are counted as incorrect. Nothing is subtracted from a score if you answer a question incorrectly. Therefore, to maximize your score it is better for you to guess at an answer than not to respond at all.
from ETS's Subject Test Taking Strategies: https://www.ets.org/gre/subject/prepare/strategy/
Is this a new change in test policy? Or is ETS straight up passing a fast one (I think that's unlikely)?