Indeed, if it is not objectionable, there should be no punishment, in which case if they could trust this there would be no issue with them signing their names to their statement.
You are right, they likely do not sign their names because they fear retribution. In other words: They do not trust that there will be no punishment. The demands to unmask them show that this risk is real. It does not follow from this that this is something that warrants punishment.
See how that works: Someone can believe - whether or not they are right - that there is nothing in it warranting punishment, and at the same time believe that there will be punishment anyway.
That is a logically consistent position to hold, and sufficient to warrant not disclosing their names, and so it is not valid to try to infer from this that they belief that they’ve done something wrong, nor is it reasonable to expect everyone who believes in a cause to consider it so important to them personally that they are willing to risk their future careers over it.
Unless you yourself have taken greater risks in the name of this cause, you have no basis for demanding of them risks you are unwilling to take yourself.