but where does it say in the books fem presenting dwarves have beards

Where does it say it in the books? Okay, let's see (and this isn't even an exhaustive list of the textual proof of all Tolkien dwarves having beards).

Amazon has the rights to the appendices of LotR which includes this:

“It was said by Gimli that there are few dwarf-women, probably no more than a third of the whole people. They seldom walk abroad except at great need. They are in voice and appearance, and in garb if they must go on a journey, so like to the dwarf-men that the eyes and ears of other peoples cannot tell them apart. This has given rise to the foolish opinion among Men that there are no dwarf-women, and that the Dwarves ‘grow out of stone” (Durin’s Folk, LotR Appendix A).

And from the Histories of Middle Earth (Amazon does not have the rights to the Histories) but it is still evidence of Tolkien's intent for female dwarves to have beards:

“The Naugrim were ever, as they still remain, short and squat in stature; they were deep-breasted, strong in the arm, and stout in the leg, and their beards were long. Indeed this strangeness they have that no Man nor Elf has ever seen a beardless Dwarf - unless he were shaven in mockery, and would then be more like to die of shame than of many other hurts that to us would seem more deadly. For the Naugrim have beards from the beginning of their lives, male and female alike; nor indeed can their womenkind be discerned by those of other race, be it in feature or in gait or in voice, nor in any wise save this: that they go not to war, and seldom save at direst need issue from their deep bowers and halls. It is said, also, that their womenkind are few, and that save their kings and chieftains few Dwarves ever wed; wherefore their race multiplied slowly, and now is dwindling” (Concerning Dwarves, The History of Middle Earth vol. 11). 

The only way you can tell male and female dwarves apart is by the fact female dwarves do not go to war.

If they [Amazon] want to argue faint sideburns are enough to count as a bearded dwarrowdam in a society that places a high value on the length of beards to the point where they would rather die than be shaven, then I would call them cowards.

I've heard people use the argument of the design of the dwarves in the Hobbit to justify short beards and while I do not wish to come back often to previous adaptations, especially those by Peter Jackson when speaking about Rings of Power, at least the choice to have a prominent character like Thorin Oakenshield wear a shorter beard is consistent with Tolkien’s other writings.

Here is what Richard Armitage says about it: 

“I needed to find a reason for this and when I read Thorin’s account of how when his grandfather and father, Thrór and Thráin, came out of the lonely mountain after the attack by Smaug the dragon, they had singed beards. This gave me the solution: he has his beard cut short, as a mark of respect to the indignity suffered by them. Perhaps if he ever gets to sit on his throne again as king he’ll grow a big old beard and tuck it into his belt, just like Tolkien wanted!” (from The Hobbit: An Unexpected Journey Official Movie Guide)

This choice of ‘mourning ritual’ can be backed up in the appendices:

“Such was the tale that Nár brought back to Thráin; and when he had wept and torn his beard he fell silent. Seven days he sat and said no word” (Durin’s Folk, LotR Appendix A).

There is no reason a dwarvish princess should not have a full beard other than to conform to modern society's idea of gendered appearances, which arguably, were not important among Tolkien’s dwarves considering males and females are said to look identical. 

You can read more about my thoughts on the show and lore (including bearded female dwarves) here.