Why are the Weasley's blood traitors?

[lily_paige_delaney]

No: HPFGUIDX 76010

Does anyone know why the Black's, Malfoy's etc would consider the 
Weasley's blood traitors when they don't seem to have committed the 
biggest crime - marriage to muggles?

OoP Ch 6 pg 105, Sirius tells Harry there is no point looking for the 
Weasley's on the family tapestry because " ...if ever a family was a 
bunch of blood traitors its the Weasley's..."

In CoS (Ch18 pg 247)Dumbldore says the Weasley's are one of "our most 
prominent and pure-blood families".

They obviously haven't intermarried so are they blood traitors just 
because Arthur has a soft spot for Muggles?  Surely not.

In CoS (Ch 4 pg 51) Lucius Malfoy says to Arthur

"...Dear me, what's the use of being a disgrace to the name of wizard 
if they don't even pay you well for it?.."  

and upon seeing Mr and Mrs Granger

"...The company you keep, Weasley...and I thought your family could 
sink no lower-.."

This begs the question - what other company do the Weasley's keep and 
what have they done to previously sink so low in Malfoy's eyes?

It can't just be Arthur's job - there must be more to it than that?  

Any ideas?

LPD