All people with Tourette syndrome have tics, which are semi-voluntary behaviors involving movement or vocalization. A small percentage have complex phonic tics, including echolalia (repeating words other people say) and palilalia (repeating one's own words). There's also a tic involving compulsively saying taboo or inappropriate words, which is called coprolalia and although rare, is the most commonly depicted tic in portrayals of characters with TS.
My uncharitable assumption is that one of the TicTock (get it?) girls saw something about a character with TS who had coprolalia and adopted it for whatever reason, but was uncomfortable swearing on Tiktok. Thus, "beans!" The other girls, who also have no understanding of TS other than television, have co-opted it because they think it makes them look like True and Honest sufferers.
It's possible that one of them developed this particular tic after reading or hearing someone else say it (palilalia) and now it's spread to the others the same way, but I don't give them that much credit.
Also, the way this particular tic is manifesting is much more similar to
klazomania than to a legitimate TS symptom. Unfortunately for the beans! gang, klazomania isn't a TS symptom and is instead observed in patients with brain damage, encephalitis, or seizure disorders. All of the "motor tics" I've seen from these girls really resemble
dystonia or
chorea much more than they do Tourette syndrome.
Also also, tics are only
semi-voluntary, meaning that they can be suppressed, although this does cause some degree of discomfort for the patient. Most adults are able to suppress the urge when it isn't appropriate to tic. I'd think a bit of discomfort is a fair trade for not smacking your kid, but what do I know. The weird, involuntary jerking is, as I mentioned, much more like a dystonia or chorea than a tic.
Tl,dr: It's because all of them are of low average intelligence and are faking based on a very limited understanding of Tourette syndrome and its symptoms.