As zarf says, it’s a bit more intuitive when you appreciate the difference between a rule, a rulebook, and an activity. Emily already explained this, but this may help a bit until you get to the relevant parts of the documentation.
Printing the banner text is an activity. An activity can have rules, but an activity is not quite the same as a rulebook, even though they’re similar. You are allowed to attach rules before (“before …”), attach rules after (“after …”), or overwrite the normal behavior for that activity (“rule for …”).
There’s a whole chapter about Activities in WWI that tries to explain the difference between activities and rulebooks, but it’s definitely not obvious when you’re just starting out. However, it does make more sense in the long run. I tend to think of activities (at least those like “printing the banner text”) as places where what’s being done is part of the lowest-level functionality provided by I6 layer – though that’s not the complete picture.
The “room description heading rule” is a part of the action-processing rules: it’s a rule within the carry out rulebook for the looking action provided by the Standard Rules. There’s another whole chapter about action processing in WWI. You can remove, replace, conditionally disable, or write a new rule more specific than this rule within that rulebook, but when you say “Rule for printing room description heading…” you’re basically asking to create a rule for a rule. That doesn’t make any sense, as you can imagine.
Where you say “I can change the action of a rule, but not replace it,” neither is strictly true. Saying " does nothing when " is changing the action of the associated rulebook, i.e. telling it to skip that rule if the condition is true. A rule can be replaced within a rulebook, if desired, but creating a new rule with specific conditions (e.g. “Carry out looking when tutorial mode is true”) does not automatically replace a similar rule with a different or no condition (e.g. “Carry out looking”).
Why some things are governed by rules and others are governed by activities is not always obvious, and may seem arbitrary, but you will get familiar with the dividing line over time. For now, brace yourself, do some more reading (especially the chapters on Action, Activities, and their code examples in the Recipe Book), and don’t get too frustrated just because I7 is big and complex – the people here are almost always willing to help.