It is reasonable that game authors, at times, need to impose some limitations on characters, not to let some class or style of character to overshadow all the others. But to put it bluntly, the fluff must cover the crunch: arbitrary mechanical limitations must be justified in the setting, however vaguely. In D&D we have always had this problem with things such as (a) Vancian magic (never justified in setting: I will write about it some day) or (b) weapon limitations. Strange as it seems, the case of clerics was, if anything, less absurd than, say, the cases of magic users and thieves. BECMI, which strictly enforced the “no sharp weapons” ban for clerics, explicitly gave a religious taboo as explanation: “you shall not shed blood”. Of course, things got awkward when you realized that the same ban was supposed to apply to clerics of all sorts of chaotic and/or evil deities: in BECMI adventures, evil clerics such as Xanathon or the Master wielded maces or warhammers, never spears or swords. In fact, one could go as far as to say that this ban is more plausible than the entirely arbitrary dichotomy which is enforced in post-2000 editions between simple and martial weapons: there, it is obvious that the only basis for the distinction is the will to handicap thieves and clerics, because it is obviously awkward to imagine that the spear (the main infantry weapon for millennia) should be regarded as “simple” as opposed to “martial”. AD&D2e had injected some common sense by saying that clerics were expected to have restricted choice of weapons, but the exact restriction was to be determined in setting, depending first of all on the respective cults. But the bans on thieves or magic users were and are entirely unjustified. Nothing explains why they should not be able to use certain weapons or, worse, to wear armour. Mind you: one reasonable way out would have been to say that if they did wear heavy armour, they could not use their special abilities (casting spells or doing furtive things). That would have made sense (Rolemaster did that with magic). But no, the authors were adamant about it: people could not use the weapons or wear armour, that was all. How to justify it in game, was up to the DM. It is nice, if somewhat amusing, that this particular flaw of the D&D system was never solved to this day, while for years people fought edition wars over attack tables vs THAC0 vs roll-over-AC. De gustibus, I say.
EDIT (5 June 2015): I had entirely forgotten (in spite of playing the game for at least two years!) that AD&D2e had penalties to thieves skills when they wore armour, as opposed to flatly forbid them to do so. Wizards, on the contrary, were never allowed to wear any armour because “they had no time to learn about armor” (sic).