Eggshells are actually very porous. Putting eggs in very salty water will see the salt get absorbed through the shell and into the white and yolk.

'Salty eggs' are often used in Chinese food...

If you're frying or baking them, you salt them before they set, rather than after they go on the plate. If you're poaching, you salt the water. If you're doing the crazy poach-in-the-shell stuff, I imagine you do something with the shell porousness, like the sibling comment.