Cato was praetor 54, not 59. Plutarch claims that Cato was dragged down from the Rostra to prison again in 55 when he opposed the lex Trebonia. Cato was already praetor-elect by then, but that still doesn't make him praetor.

Cato's supposed imprisonment in 59 is well-attested yet is extremely doubtful. Plutarch mentions it twice in the Cat. Min. yet his two accounts do not agree--at Cat. Min., 31 he says it occurred in the senate (c.f. Gellius 4.10.8 and V. Max., 2.10.7) but at Cat. Min., 33 (c.f. Appian, B.C., 2.11) say that it happened at a contio. A legend arose around Cato even before he was dead, and this sort of contamination is not unknown. How many times Cato was arrested, and under what circumstances, is a very debated question.

Strictly speaking the argument that Caesar could not have his apparitores arrest Cato seems weak. As consul Caesar had the power of coercitio over Cato, who was merely a privatus. Coercitio had its limits: it was subject to provocatio and tribunician intercession. But Cato does not appear to have used provocatio in any of the incidents in the texts, and no tribune seems to have come to his aid. Nonetheless, Caesar's actions were certainly of questionable nature, even if not necessarily outright illegal. The powers of coercitio were increasingly being restricted in the first century, eventually becoming quite heavily regulated by a lex Julia de vi some years later. If Cato was really arrested (Morstein-Marx, for example, actually thinks he was allowed to speak at the contio) Caesar would have been conjuring up unpleasant memories of the abrogation of Octavius' magistracy and Cicero's arrest of the Catilinarians, among whose number was a sitting praetor.