I may be wrong, but I think it has something to do with the fact that damage to a painting is usually due to the environment is which it is displayed/stored and damage to a card is usually due to abuse.
Nobody abused the painting, it deteriorated over time. However, pinholes, writing, creases, paper loss, etc. on a card is usually due to abuse.
It's not acceptable to repair abuse, it's acceptable to repair natural deterioration. At last that's my $.02
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