Young man, do not take that flippant tone with me (raps ruler on desk). Those novice developers failed to RTFM. If you want a framework that produces secure applications for inexperienced developers who do not read the fine manual, you are setting a laudable goal for Rails, but I hardly think that “Secure even for people who are 1. novices and 2. compound their ignorance by refusing to RTFM” is synonymous with “security problem.”
If you open your history book to when Rails was first becoming popular, there were many people grumbling that because it was built on top of a dynamic language that it was vulnerable to all sorts of bugs caused by novice developers getting their types wrong. The argument at the time was that Rails provided a certain type of freedom and power at the expense of certain safeguards. The same argument was played out when people started noticing that monkey-patching run amuck caused certain problems.
I agree with you that this is a design choice. If you want to say that you disagree with the choice as it was originally made, I agree with that too. I won’t say that I would have made a different choice back when that feature was first baked in, I did not write the framework. But here we are today, and it seems like a very good time to make a different choice.