yaml indentation oddity

capture.txt

$ cat load.py
import yaml
import sys
f = open(sys.argv[1], 'r')
y = yaml.load(f)
print(y)
$ 
$ cat y1.yaml
a:
  - b: |
      one
      two
  - c: |
      three
      four
$ 
$ cat y2.yaml
a:
  - b: |
    one
    two
  - c: |
    three
    four
$ 
$ python load.py y1.yaml
{'a': [{'b': 'one\ntwo\n'}, {'c': 'three\nfour\n'}]}
$ 
$ python load.py y2.yaml
Traceback (most recent call last):
  File "load.py", line 4, in <module>
    y = yaml.load(f)
  File "/usr/local/lib/python2.7/site-packages/yaml/__init__.py", line 71, in load
    return loader.get_single_data()
  File "/usr/local/lib/python2.7/site-packages/yaml/constructor.py", line 37, in get_single_data
    node = self.get_single_node()
  File "/usr/local/lib/python2.7/site-packages/yaml/composer.py", line 36, in get_single_node
    document = self.compose_document()
  File "/usr/local/lib/python2.7/site-packages/yaml/composer.py", line 55, in compose_document
    node = self.compose_node(None, None)
  File "/usr/local/lib/python2.7/site-packages/yaml/composer.py", line 84, in compose_node
    node = self.compose_mapping_node(anchor)
  File "/usr/local/lib/python2.7/site-packages/yaml/composer.py", line 133, in compose_mapping_node
    item_value = self.compose_node(node, item_key)
  File "/usr/local/lib/python2.7/site-packages/yaml/composer.py", line 82, in compose_node
    node = self.compose_sequence_node(anchor)
  File "/usr/local/lib/python2.7/site-packages/yaml/composer.py", line 111, in compose_sequence_node
    node.value.append(self.compose_node(node, index))
  File "/usr/local/lib/python2.7/site-packages/yaml/composer.py", line 84, in compose_node
    node = self.compose_mapping_node(anchor)
  File "/usr/local/lib/python2.7/site-packages/yaml/composer.py", line 127, in compose_mapping_node
    while not self.check_event(MappingEndEvent):
  File "/usr/local/lib/python2.7/site-packages/yaml/parser.py", line 98, in check_event
    self.current_event = self.state()
  File "/usr/local/lib/python2.7/site-packages/yaml/parser.py", line 428, in parse_block_mapping_key
    if self.check_token(KeyToken):
  File "/usr/local/lib/python2.7/site-packages/yaml/scanner.py", line 115, in check_token
    while self.need_more_tokens():
  File "/usr/local/lib/python2.7/site-packages/yaml/scanner.py", line 149, in need_more_tokens
    self.stale_possible_simple_keys()
  File "/usr/local/lib/python2.7/site-packages/yaml/scanner.py", line 289, in stale_possible_simple_keys
    "could not find expected ':'", self.get_mark())
yaml.scanner.ScannerError: while scanning a simple key
  in "y2.yaml", line 3, column 5
could not find expected ':'
  in "y2.yaml", line 4, column 5
$