under our current laws you can only purchase seed from a gov't approved vendor. of which there are near zero. anything else is as illegal as it used to be.
hydro shops selling seed would open themselves up to an expensive world of pain.
seeds purchased from online sellers are acquired through a loophole in the law dating back to pre-legal. those seeds are ok to purchase as a novelty item, but considered illegal once they are germinated.
the law is still on the books, but it is not being enforced. grow shops stay away, as they can still get in trouble, unless they got the seed or clones through a gov't approved supplier.