[c-nsp] Why is implicit-null advertised only for connected/summary routes?

[Everton da Silva Marques]

From:
http://www.ciscopress.com/articles/article.asp?p=680824&seqNum=2&rl=1

  PHP is the default mode in Cisco IOS. In the case of
  IPv4-over-MPLS, Cisco IOS only advertises the implicit
  NULL label for directly connected routes and
  summarized routes.

Question: What is the reasoning behind restricting
implicit-null to connected and summarized routes?
Can anyone please shed some light on why it
should not include static routes as well?

Thanks,
Everton