Imagine if you will an email program in which Reply All is not only the default reply option but also the only reply option. Most annoying thing ever? Nope: a text application that works like that would be the most annoying thing ever--and as it happens, that's exactly how the iOS Messages application works: if you receive a group message (or group MMS or group SMS or group iMessage message or whatever else) on an iOS device, you can reply to all the recipients of that message but you cannot reply directly to only the sender*. I'm shocked that Apple--that anyone--would implement Reply All as the default for replying to a group message and I'm even more dumbfounded that this is the only reply option available: surely it is not unimaginable to Apple that some users might value the ability to reply back only to the sender of any given group message as a per-message option.** But hey: mistakes and bad decisions are made by all of us in life; it's not the end of the world! But it sure would be nice if it were fixed: Apple, please recognize the disaster that this implementation is (if by no other means than by way of all the annoying group text replies that your employees themselves surely receive as a result of this implementation) and fix it. My thanks!
*Workaround (albeit one that, as well as I can tell, pretty much no one ever uses): create a brand new message to only the sender. The keyword there is "workaround": it is not by any means a justification for not enabling a direct "reply to sender" functionality in the Messages app.
**There is apparently the ability to turn off group messages at a global level in iOS. This apparently prevents you from being able to send group messages; I do not know if it prevents you from receiving group messages or not. Going back to the earlier email analogy, this would be like an email program that won't let you put more than a single recipient on any given email and potentially like an email program that won't let you receive emails unless you're the only recipient on them.