I couldn't agree with you more.
If we issue a Royal Pardon for Turing, then logically we have to issue pardons for everyone who was convicted under the mediaeval law in force at the time.
If we issue a pardon for everyone convicted under that law, then logically we should issue pardons to people convicted under *every* law now seen to be barbaric. Pardons for the shot shell-shocked accused of cowardice in WW I; pardons for all the suffragettes; pardons for all the children hanged for stealing a loaf; the list is endless.
Then, once we legalise the consumption of cannabis, we have, logically, to pardon all those convicted under the present law. And so on.
This idea is called "re-writing history". Orwell satirised it in '1984'.
People, the damage is done! Although all these laws were abominable, they were the law at the time these injustices happened, and the injustices happened within a valid, if barbaric, rule of law.
We have to learn, improve, and move on. Pardons, and even apologies, just don't make sense.