So the Germans were only greeted warmly on ex-Polish territory, is that what you're claiming?
You'll always be able to find at least some people who will be willing to collaborate wherever you go, but
on average the Germans arrival was only celebrated with mass celebration on either ex-Polish territory or the Baltic States (in the latter case, there were even pro-German uprisings). Further east of that, the prevailing reaction from most Soviet citizens to the arrival of German forces ranged from apathetic to hostile.
It's not like this was a unique phenomenon to the Soviets. There are innumerable examples (in no small part because the overwhelming majority of rulers throughout history have been oppressive autocrats of some variety), but we can start off with Nazi Germany, which was a totalitarian state that took away the liberties of its citizens, crushed their consumer economy, killed half a million of them directly, and got another 4.5 to 5 million of them killed in a war of choice which it started. And right to the end the great bulk of Germans carried out its policies
passionately even when they otherwise professed to hating it. Similarly, the pre-war Polish governments slide into authoritarianism during the 30s also caused serious discontent. There was a mass strike by millions of peasants in 1937 which the government put down by violently oppressing the peasants. This solved nothing, and the unrest was still bubbling in the lead up to WWII. And yet those same peasants put aside their disagreements with the government and formed a united front against the Germans in 1939, because that's what people usually
do when the government they believe is theirs is attacked by an outsider, whether they like the government or not. The cases where they don't are so notable precisely because they're the exception. The power of a state imbued with legitimacy by it's citizens to convince those same citizens to do what it wants should not be underestimated.
How did the mostly Belorussian area act, when 'liberated'
from Poland under Soviet administration ? half as many flowers?
Probably about as welcoming as the pre-'39 Polish territories of western Belorussia, which I can't imagine would be that much different from Western Ukraine or the Baltics...