The purpose of grep is to filter a list. It does so by evaluating a piece of code for each item in the argument list. It wants a yes/no answer from the code it executes, so scalar context is warranted.
The purpose of map is to transform a list. There is no correlation between in the size of the input list and the size of the output list. A given element may be transformed into zero, one or many elements, so list context is warranted.
To tie this back to the subject at hand, this is basically how all operators work:
- If an operator requires exactly one value for an operand (such as either operand of the scalar assignment operator), the operator will impose a scalar context on that operand.
- If an operator requires multiple values for an operand (such as either operand of the list assignment operator), the operator will impose a list context on that operand.