They certainly weren't forced to accept it, but it was a very reasonable deal to make. I mean non-aggression, without the secret clause :) Again, imagine you're in Stalin's shoes - it is just natural to assume the West will leave the USSR fight Germans alone. Heck, it was the ONLY thing to assume. Of course, Stalin being Stalin, he quickly figured he could get something "more" than peace :)
Turns out, Hitler wanted non-aggression even more, as he was scared that USSR had the military alliance with the West, or was very close to making one (he was wrong). Stalin calculated that Germans would accept an extra clause, a pretty harsh one for German ambitions in the East, as a prerequisite to non-aggression (he was right), which allows the USSR not only to delay war, but also get a "piece of the pie" in Poland, Bessarabia, Baltics and Finland.
I like the argument made by William Shirer in his "The rise and fall of the Third Reich", which Stalin could've used to justify his actions:
"If Chamberlain was right and honourable in appeasing Hitler in September 1938 by sacrificing Czechoslovakia, was Stalin wrong and dishonourable in appeasing the Fuehrer a year later at the expense of Poland, which had shunned Soviet help anyway?"