I haven’t looked at the context, and I’m not sure I understand this properly, but the “rule” that a subordinate clause in indirect speech (acc. + inf.) takes a subjunctive verb comes into play where the subordinate clause is part of reported speech, and the writer is not necessarily taking responsibility for the accuracy of the statement in the subordinate clause. In this passage, S. is stating a fact he regards as true and accurate. The subjunctive makes it clear that the subordinate clause is part of what the speaker (as opposed to the writer or narrator reporting the speaker’s words in the inf + acc construction) said, not a comment or clarification by the writer/narrator.
“Come on, don’t you even understand this: that bad examples come back to bite those who make them?” Or something like that.
S. here is stating a fact that he regards as true, not a claim or statement made by his (imaginary) interlocutors: in fact, it’s a truth they’re supposedly unaware of. So Seneca uses the indicative, not the subjunctive, in the relative clause.
Allen & Greenough, sec. 583 and 583a:
- A Subordinate Clause merely explanatory, or containing statements which are regarded as true independently of the quotation, takes the Indicative:—
quis neget haec omnia quae vidēmus deōrum potestāte administrārī; (Cat. 3.21), who can deny that all these things we see are ruled by the power of the gods?
cûius ingeniō putābat ea quae gesserat posse celebrārī; (Arch. 20), by whose genius he thought that those deeds which he had done could be celebrated. [Here the fact expressed by quae gesserat , though not explanatory, is felt to be true without regard to the quotation: quae gessisset would mean, what Marius claimed to have done.]
[*] Note.–Such a clause in the indicative is not regarded as a part of the Indirect Discourse; but it often depends merely upon the feeling of the writer whether he shall use the Indicative or the Subjunctive (cf. §§ 591-593).
[*] a. A subordinate clause in Indirect Discourse occasionally takes the Indicative when the fact is emphasized:—
“factum êius hostis perīculum … cum, Cimbrīs et Teutonīs … pulsīs, nōn minōrem laudem exercitus quam ipse imperātor meritus vidēbātur ” (B. G. 1.40) , that a trial of this enemy had been made when, on the defeat of the Cimbri and Teutoni, the army seemed to have deserved no less credit than the commander himself.
http://www.perseus.tufts.edu/hopper/text?doc=AG+583&fromdoc=Perseus%3Atext%3A1999.04.0001