This does not refer to a specific set of laws, but echoes Hesiod's Theogony, the genealogy of the Greek Gods, and the Gigantomachy.
It's called an "old" law (the text doesn't actually contain that word, see below) because the decree stem from the time of Cronus' reign, so before Zeus defeated Cronus and created the Greek Pantheon (which includes Pallas Athena). Zeus' laws are described as just in Theogeny whereas Cronus' rule is described as cruel.
As to the apparent contradiction on the punishment, blindness and sight refer to two different things here. In Calimachus' fifth hymn, Athena blinds Tiresias:
And he stood there speechless; for pain glued his knees and
helplessness stayed his voice. But the nymph cried: “What has thou
done to my boy, lady? Is such the friendship of you goddesses? Thou
hast taken away the eyes of my son. Foolish child! Thou hast seen the
breast and body of Athena, but the sun thou shalt not see again.
When Charilco pleads with Athena, she explains the reason for the punishment, an edict by Cronus:
ἆ καὶ ἅμ᾽ ἀμφοτέραισι φίλον περὶ παῖδα λαβοῖσα
μάτηρ μὲν γοερᾶν οἶτον ἀηδονίδων
ἆγε βαρὺ κλαίοισα, θεὰ δ᾽ ἐλέησεν ἑταίραν
καί νιν Ἀθαναία πρὸς τόδ᾽ ἔλεξεν ἔπος
‘δῖα γύναι, μετὰ πάντα βαλεῦ πάλιν ὅσσα δι᾽ ὀργὰν
εἶπας: ἐγὼ δ᾽ οὔ τοι τέκνον ἔθηκ᾽ ἀλαόν.
οὐ γὰρ Ἀθαναίᾳ γλυκερὸν πέλει ὄμματα παίδων
ἁρπάζειν: Κρόνιοι δ᾽ ὧδε λέγοντι νόμοι:
ὅς κε τιν᾽ ἀθανάτων, ὅκα μὴ θεὸς αὐτὸς ἕληται,
ἀθρήσῃ, μισθῶ τοῦτον ἰδεῖν μεγάλω.
Noble lady, take back all the words that thou hast spoken in anger. It
is not I that made thy child blind. For no sweet thin is it for Athena
to snatch away the eyes of children. But the laws of Cronus
order thus: Whosoever shall behold any of the immortals, when the god
himself chooses not, at a heavy price shall he behold.
Sight refers to Athena's gift of clairvoyance:
Therefore, O comrade, lament not; for to this thy son – for thy sake –
shall remain many other honours from me. For I will make him a seer to
be sung of men hereafter, yea, more excellent than any other. He shall
know the birds – which is of good omen among all the countless birds
that fly and what birds are of ill-omened flight. Many oracles shall
he utter to the Boeotians and many unto Cadmus, and to the mighty sons
of Labdacus in later days. Also will I give him a great staff which
shall guide his feet as he hath need, and I will give him a long term
of life.
