Law 2.04.07 - The covering material should be used as it has been authorised by the ITTF without any physical, chemical or other treatment, changing or modifying playing properties, friction, outlook, colour, structure, surface, etc.
That is the rule. It explicitly states that the topsheet and the sponge can not be treated in any way in order to change the playing characteristics. It is pretty clear.
Does it make sense to follow the rule? That is a different question, but the question itself has a very clear answer. The rule specifically states that the sponge and rubber should be used as they come out of the packaging from the manufacturer. That doing anything with the intention of changing the playing characteristics, after the rubber has left the manufacturer, is against the rules.
It does seem to be about $$ rather than about anything actually worth paying attention to, but, it is a rule and it is specifically stated.