For game developers, security against hacks is a matter of ROI. And we know that it's usually much easier for hackers to find a few holes than for us to plug all of them, so we often go for the low hanging fruit, rely on players policing themselves, and especially in the F2P arena, disregard players using a hack as players who weren't going to pay anyway.
Of course different games have different considerations in this regard. Consider Draw Something, where the game itself provides the ultimate built-in cheat: draw the word the other person is supposed to guess!
Yes, I can certainly appreciate that, but I think there is a minimum you can aim for without going too far out of your way. I could go into detail, but I think I'd be listing off things you already know.
Of course different games have different considerations in this regard. Consider Draw Something, where the game itself provides the ultimate built-in cheat: draw the word the other person is supposed to guess!