I am not a lawyer, and as such wont have anything to say about how may for shall struck me as odd when reading it in this context.
But I am a native German speaker and we distinguish the verb mögen in indicative ich mag (I like, I may want to), subjunctive (Konjunktiv I) ich möge and, I guess you would also call it subjunctive (Konjunktiv II) ich möchte. The precise details of these modi are immaterial, here, but it's interesting that möchte (ca. "might") has come to be understood as a separate stem, not anymore related to mögen, but without a form for, or without a practical use for, the infinitive, probably for thousands of years now. It's common consensus that the stem was the same though, which is what I will go with, though I'm incubating doubts.
The point being, "Er möge Kaution annehmen" has the obligatory sense as in "He may take bail". This does sound a bit old fashioned. The apparently sarcastic tone is used colloquially as well, Du möchtest den Müll raus bringen "you may bring out the trash", perhaps as a variant of an archaic, high council like mögest, but built from the preterite mochte, thus implying precedent. This is showing how the different moods and aspects are deeply intertwined.
Proto-Indo-European as reconstructed in the articles of wikipedia had subjunctive and optative mood. One of these was built with an *-iH, *-ey suffix, and then some, notwithstanding Schwebelaut. Before or in Middle High German, terminal -i caused Umlautung in the nucleus (Sturm, stürmisch -- storm, stormy), depending on several factors.
Clearly, in English this ending would be subsumed by y when the velar g, still visible in Old English magan, became however lenited to may. Henceforth, the grammatical difference was lost as in many other cases of erroding English morphology. Whether the meaning was nevertheless kept in good memory, as in you may go now, or reinvented by statute in the 18th c. as other opinions suggest, I don't know.
The precise interpretation of the different modi and aspects is not always clear. If you said the Sherrif can take bail, this might be construed as a statement of fact, irregardless of the actuality, whereas Germann kann sein chiefly means "maybe" in colloquial speech, largely indistinct from könnte sein "could be", though in legalese, as far as I'm aware, the former leaves the decision to the discretion of the administrator subject only to moral obligations, whereas the latter would strictly imply modus irrealis and as such cannot confer obligation. Similarly ich will "I want", ich werde "I will, I become" and English you will! show's a striking difference. mag sein is literally "may be", too, but das möchte wohl sein (doesn't translate) again plays on the preterite, ca. "that's well expected, anything else would be far from acceptable" (as if that must be, but where to stick the well?). I mean to say, there were inherent uncertainties that lead to the errosion of this grammatical system. The optative is perhaps still seen in I wanna, while the usual example of optative aspect is lemme (or rather let's), but as a grammatical function it was already lost in almost all descendants from Indo-European long ago.
I want to say this has a desiderative aspect, which, when coming from an authority, implies obligation to the subordinate, when success of the order is in their might at all (in der Macht stehen; also cp. Möglichkeit "possibility", möglich "maybe", possibly conflated with machbar "make-able, feasable"). But the difference between "I want to go" or "Don't you want to go now!?!" is quite obvious, so I don't quite know what to call either of both aspects.