Yahoo mail and Gmail policy explanation?

Hi guys, hope you can explain something for me!

Neither yahoo mail nor gmail seem to reject incoming mail if there is no DNS 
record at all for the envelope sender address, allowing spammers to just use 
completely fake domains.

Does anyone know why they might choose to have that policy? Is there any 
legitimate reason for allowing incoming mail from a fake domain? It doesn't 
even get blocked when you turn on the spam filter!

I understand the argument that if all servers blocked mail from fake 
domains, and every legitimate domain used SPF, then spammers would have a 
harder time finding sender addresses to use, but doesn't this policy from 
yahoo and gmail compromise the ability of SPF to contribute to the overall 
reduction of spam?

Thanks in advance,
Martin