OTT samba users query

[dave s]

OK I have a samba system working fine on my 2 x (k)ubuntu systems and work XP 
Laptop. I have read the manuals, configured it and it does what I want. My 
question is

If someone needs to access a share on my Samba server, they must be both a 
valid user of the system and a valid Samba user. The need for them to have a 
smbpasswd Is obvious. 

But why does samba insist that they are also a valid user on the linux 
system ? sbmpasswd -a will not work until they are. 

Is it due to something in the inner workings of samba or is there an obvious  
reason for it that I have just plain missed ? 

Can anyone enlighten me

Cheers

Dave