why do we invoke print after importing print_function (in Python 2.6)

The reason is that when you import from __future__ you're really just setting a flag that tells the interpreter to behave a bit differently than usual -- in the case of print_function, the print() function is made available in place of the statement. The __future__ module is thus "special" or "magic" -- it doesn't work like the usual modules.

print_function is a FeatureName not be confused with the print built-in function itself. It is a feature that is available from the future so that you can use the built-in function that it can provide.

Other Features include:

all_feature_names = [
    "nested_scopes",
    "generators",
    "division",
    "absolute_import",
    "with_statement",
    "print_function",
    "unicode_literals",
]

There are specific reasons as when you migrate your code to next higher version, your program will remain as such as use the updated feature instead of the __future__ version. Also if it were function name or the keyword itself, it may cause confusion to the parser.