It's not a requirement of that category of magic items as a generalization. Much like you can have a wand which casts spells as a bonus action, or a magic action, or even a 1 minute casting time in theory (though I don't recall such a wand).
However, it's a requirement of that specific magic item, if that specific item requires the magic action to trigger it.
Examples:
Wand of Radiance: "A creature that is attuned to Radiance can use a bonus action...to cast the enhance ability spell, choosing itself..." As this specific wand does not require a magic action to activate it, it cannot work with fast hands.
Wand of Magic Missile: "you can use an action [now called a Magic action] to expend 1 or more of its charges to cast the magic missile spell from it. For 1 charge, you cast the 1st-level version of the spell. You can increase the spell slot level by one for each additional charge you expend." As this specific wand does require the magic action to activate it, it can work with fast hands to reduce the casting time to a bonus action.
No specific description of a Spell Scroll says that it takes a Magic action to cast as far as I can see.
So while the category of magic items "Scrolls" does not require that you take the Magic action, a specific scroll does require you to take the Magic action. The distinction of "to cast" does not seem relevant as all magic items which we all agree are supposed to be covered by Fast Hands, like a Wand of Magic Missile, also uses the term "to cast [the] spell." Most magic items which could be used with Fast Hands cast a spell, and the magic item user is designated as the caster most of the time.
We don't all agree. I don't know what the
wand of magic missiles will be like. I don't have the '24 DMG yet. I could see it being a situation where the designers allow it to be covered with Fast Hands ("take the Magic action" being part of its activation description), but I could also see it NOT being allowed there (wands are a lot like scrolls and staves in that they typically require the spell to be on your class's spell list, and so may be designed much like scrolls in that it enhances your casting of the spell, rather than taking the Magic action to generate the effect). Because the wand currently has no attunement restrictions, I'd wager it'll be the former, honestly. Neither outcome suggests that
spell scrolls follow the same logic, though.
A spell scroll of magic missile, like a wand of magic missile, both require the Magic action by that specific item. Both "cast the spell." It's not tied to a category of magic items, which is just a title used to organize magic items. It's tied the requirements of that specific item.
A
spell scroll is a specific item and it doesn't require the Magic action. It requires casting a spell. A
wand of magic missiles or any other item that duplicates the effects of a spell may or may not require casting the spell, as is appropriate for that item. Like, I could see a world where the
wand of entangle, the
staff of fire and the
staff of charming require casting the spell, but the
cloak of invisibility, the
wand of wonder and the
necklace of fireballs require taking the Magic action. In that case, you could use Fast Hands with the latter trio, but not with the former trio.
And I imagine that this (unnecessarily subtle, IMO) distinction will largely hinge on if the item is intended to be used by spellcasters to more easily cast spells (in which case I'd wager you'll mostly need to follow the rules for spellcasting, like with
spell scrolls and most staves and a lot of wands), or it's an item mostly intended for a general audience to reproduce the effects of spells (in which case I'd imagine you'll find a lot of Magic Action activations, like the
wand of wonder or the
wand of magic missiles).
I don't imagine the design intention was to give a specific multiclass combo exclusive access to a better version of Quicken Spell and a better version of Sneak Attack all in one combo, so RAI seems pretty clear to me. RAW, I can see the argument that casting a spell requires the Magic action, so
de facto any item that requires casting the spell also requires using the Magic action, but there's that leap in that spell scrolls don't themselves require the Magic action.
I do suspect this will clarify a bit when we have more items to work with and we can see if the designers did, indeed, care about the distinction between using the Magic action to cast a spell (which may be with magic item assistance) and using the Magic action to activate a magic item in some way (which may include causing it to cast a spell).