The political reasons of both France and Britain are well explained in other answers, so I just stick to the legal matter.
France was not legally obliged by any pact to attack Soviet Union or to send troops to Poland to help. The 1921 Franco-Polish treaty specified the extent of help, which amounted to keeping the communication lines free between France and Poland (France and her Eastern Allies, 1919-1925). The 1939 pact, already ratified on September 4, was strictly against Germany, and had no provisions against Soviet Union (Britain, Poland and the Eastern Front, 1939).
Britain, on the other hand, was legally obliged to attack Soviet Union, literally "at once" and to provide "all the support and assistance in its power", per the 1939 pact. There was no legal trick that allowed Britain to avoid this. Britain recognized Poland as a country, and the pact obviously didn't require Poland to be recognized by attacking enemy. The pact did not require Poland to declare war on attacking enemy. Moreover, ambassador Raczyński requested such help from Britain as soon as Soviet Union attacked, and Halifax declined without any meaningful reason (Britain and Poland 1939-1943: The Betrayed Ally). Halifax said, 'As regards Soviet aggression we were free to take our own decision and to decide whether to declare war on the USSR or not.' (Britain and Poland 1939-1943: The Betrayed Ally)