You can push those rolls because they are reaction rolls not opposed rolls in that sense.
In a normal opposed roll, the "defender" is passive, he spends no action doing so, so he can't push his roll either. In combat, the defender do spend an action dodging or parrying. He is also active.
Is this stated somewhere explicitly, as in the Reactions section it is mentioned only reactive action, not rolls.
It is not spelled out directly (it probably should) but if you read the combat chapter you will get an understanding how it is meant to work. It helps of course to know where the idea for this rule comes from (MYZ).
So, the rules say that Dodge and Parry are called reactive actions because they break the initiative order but still counts towards your two actions for the round. This means that both the attacker and the defender are active during an attack. The defender just happens to do this not on his normal initiative turn. If we look at social conflicts, one of the few non-combat actions that you can do in a conflict that is also an opposed roll, the rules state that it only counts as an action for the "attacker", not for the "defender".
When it comes to Dodge and Parry, the defender is actively doing something and if you actively doing something it makes sense that you also can push yourself. If someone is just scouting and he happens to be sneak attacked for example, he is not actively doing something, so it is logical that he is not able to push. He doesn't know that he will be sneak attacked in a moment so why would he all of a sudden start to focus extra hard on his scouting (or whatever pushing scouting would be).
This is just my interpretation of this of course. If you don't like your players to push dodge and parry, don't let them. Although, a very lethal game will be a hell of a lot more lethal if you don't.