It has nothing whatsoever to do with PADI, per se. I'd feel the same way about NAUI (all of my college courses); SSI; or the late-lamented YMCA, where I began -- and I'm none too damn fond of GUE, since they don't allow for upstream regulators (and we just gave my niece a nice set of Poseidon XStreams for Christmas), prior to her coursework, who's now headed to NAUI instead, who doesn't give a rat's ass what you use.
When I began, doubles -- independent or otherwise -- were the still most common arrangement, though, honestly, on their way out; and it seemed particularly odd to go with just a single cylinder, some years later -- but I didn't need a f**king course, to tell me that . . .