They must have copied it from some website/book online
If they have copied from some websites there should not be any reasons for them not to provide the online links. So, in those cases they should (at least they can).
But, I many times copy from PDFs and also hard copies of books in answers. In those cases, giving online links isn't possible. At the most, I can provide some links from which readers can know what is this scripture I'm quoting from.
So, if you want the proposed system to be made mandatory, then it's problem for some users like me.
For example, I'm quoting the following from a PDF of Mahanirvana Tantram:
At the time of Ekoddishta Shraddha cooked rice and Pinda should be
given whilst facing south. The rest of the ceremony is the same as
that which has been already described, with the exception that sesamum
should be substituted for barley (71). The peculiarity in Preta
Shraddha is that the worship of Ganga and others is omitted, and in
the framing of the Mantra the deceased should be spoken of as Preta
whilst rice and Pindas are offered to him (72). The Shraddha performed
for one man is called "Ekoddishta." In offering Pinda to the Preta,
fish and meat should be added (73). O Mistress of the Kula! know this,
that the Shraddha which is performed on the day following the end of
the period of uncleanliness is Preta Shraddha (74). If there is a
miscarriage, or if the child dies immediately on birth, or if a child
is born or dies, then the period of uncleanliness is to be reckoned
according to the custom of the family (75). The period of
uncleanliness in the case of the twice−born is ten days (for
Brahmanas), twelve (for Kshatriyas), and a fortnight (for Vaishyas);
for Shudras and Samanyas the period is one month (thirty days) (76).
On the death of an Agnate who is not a Sapinda, the period of
uncleanliness is three days, and on the death of a Sapinda, should
information of it arrive after the period prescribed, one becomes
unclean for three days (77). The unclean man, O Primordial One! is not
entitled to perform any rite relating to the Devas and the Pitris,
excepting Kula worship and that which has been already commenced (78).
Persons over five years of age should be burnt in the burning−ground,
but, O Kuleshani! a wife should not be burnt with her dead husband
(79). Every woman is Thy image−−Thou residest concealed in the forms
of all women in this world. That woman who in her delusion ascends the
funeral pyre of her lord shall go to hell (80). Kalika! the corpses
of worshippers of Brahman should be either buried, thrown into running
water, or burnt, according as they may direct (81). Ambika! death in a
holy place or a place of pilgrimage, or near the Devi, or near the
Kaulikas, is a happy one (82). He who at the time of his death
meditates on the one Truth, forgetful of the three worlds, attains to
his own Essential Being (83)
The bold portion is quoted in one of the questions you've linked. In this case, I can't give online links (although if I try I can get links too I guess from Google).