While both Britain and France had defensive pacts with Poland which they honoured by declaring war on Germany after the German invasion, the British pact had a clause that actually specified that "attacked by a European power" refered specifically to Germany. The Polish ambassador to Britain, Edward Raczyński, did try to remind the foreign secretary of the defensive pact and was rather bluntly told that whether or not Britain declared war on the Soviet union or not was British business.

The pact with France had never enjoyed wide spread support within the French military leadership and the Franco-Polish relations had suffered quite a lot in the 20's and 30's. There was a plan to supply the remaining forces in southern Poland with air support but this was never carried out as it was deemed a futile effort.

To put things another way, by the time the Soviet Union invaded Poland, Poland was already done for. There was really no possible way Britain or France could have made a difference at this point. Rather than declare war on the Soviet Union the Allies opted for the more pragmatic approach of seeing the Soviet invasion as a stop to German territorial ambitions in the east. Soviet was also considered a potential ally in the war against Germany at this point (which they did eventually turn out to be).

The Soviet Union also had more easily accepted casus belli, that of protecting the non-Polish minorities in the Soviet occupied regions of Poland. The Baltic and the Outbreak of the Second World War contains a pretty detailed analysis of the Polish invasion and the diplomatic efforts surrounding it.