cogadh: Allowing that exception leaves an opening that can be exploited exactly as I described above.
The only scenario you described was this and it only contains "buying" actions and no redeeming actions so clearly it does not apply:
"Account hijacked, real human user buys something and passes the captcha, then sets a script/bot running that buys multiple codes automatically. Without that extra human challenge, the system is easy (well, easier) to exploit and do a lot of damage."
Also: No one (at least not me) is arguing about many consecutive redeems of bought codes. The case here is very clear and very simple: You get a "free" game due to buying other games, you have to pointlessly enter some captcha. I really see no way to argue the sensibility of this captcha check. And considering that (due to autumn sale) this may well be the most frequent redeem scenario it obviously matters significantly and is not some rare exceptional use case.
cogadh: Why you fail to comprehend this very simple scenario is beyond me, but it is obvious that you are either unwilling or unable to do so.
Or maybe it is because this highly damaging abuse scenario exists only in your mind.