Why does python let me do this?

crazy.py

# Python 3.4

# A nice class
class Person:
  def __init__(self, name):
    self.name = name
    

# A nice function
def speak():
  print("Woof!")

# A nice person  
jeff = Person("Jeff")

# A bad thing!
jeff.s = speak
jeff.s()    # => "Woof!"