These are all pretty much correct. It started in England. Initially, it was just a requirement of the British government in the 17th century to place the name of the manufacturer on Ace of Spades - this was probably due to disputes over stolen design ideas, which is ironic, when you consider that the British designs of the time were "borrowed" from the dominant design of Rouen, France, the leading exporter of playing cards to Britain. By 1711, when stamp taxation was extended to playing cards, printers originally had to make a inked hand stamp on the card to indicate the tax was paid. Roughly five decades later, the hand stamp was replaced by an elaborate Ace of Spades design - the more elaborate, the more difficult to accurately forge. About 60 years after that, the tax marking was changed to the wrapper in which the deck came (this was before the tuck box), but the tradition had been established and most printers kept making elaborate Ace of Spades designs as a sort of hallmark of their brands.
The Ace of Spades tradition does vary - while elaborate Aces of Spades are part of what we now call the International Standard, there are some countries where taxes were indicated as having been paid by a stamp to a different card. If I remember correctly, in Spain and Argentina, tax stamps were placed with ink hand stamps on the Four of Clubs, and just as with the Ace of Spades, decks from those countries today will often bear a manufacturer's mark on that card instead, now that the taxes have been repealed.
We still place stickers over the flap of a sealed pack of playing cards at the factory, but this, too, is a tradition that was borne out of government intervention and taxation. In the early 20th century, US and Canadian playing card manufacturers used special stamps issued by their respective governments to indicate that taxes had been paid on that particular pack of playing cards before it could be sold. Additionally, some individual states in the US charged a tax on playing cards and issued stamps of their own - the state of Alabama still charges a tax on playing cards to this day and issues stamps that much be purchased and applied to decks of cards over the flap opening before the cards can be sold in that state to show payment of the tax.
Eventually, the US Internal Revenue Service (which charged the tax) permitted manufacturers to create their own stamp designs with their company names on them to indicate payment of the tax. By 1965, the tax was repealed, but most manufacturers continued using a company-issued non-tax seal because their their customers had become used to seeing them - they were typically decorated with a company logo or brand name, acting as a promotion of the company name.
For a brief period of time in the early 2010s, many custom deck designers stopped using stickers for deck seals. They felt that the designs were ugly and clashed with the work they were creating, plus their customers hated the residue left behind when removing the stickers to make for a more attractive presentation while using the cards. This, however, didn't last for long - printers started to offer designers a chance to create custom-designed deck seals, and designers leapt at the chance. Now, some designs are even serially-numbered to indicate the number of decks made and which number deck you possess. Some seals are added at the manufacturer to the box flap opening while others are added after the deck's been created, either without a cellophane wrapper or on the cellophane itself, acting as a secondary seal to the one applied to the box.
Why is the Ace of Spades the most decorated card in the deck?
You ask this question, yet you have a beautifully decorated Spade as an avatar? ? ?
Hey, go easy on him, he's new around here! Actually, it was a pretty legitimate question, one we haven't discussed around here before to the best of my knowledge. We probably just assumed everyone knew and moved on from there. Obviously, though, that wasn't the case.