If it was an annexation in the same quality as what the Nazis did, then why did Poland not resist the invasion (which they did against Nazis) and why did the Allies not declare war against the USSR?

followed by mass arrests and deportations

There were arrests and deportations, true. And not just that, there were also summary executions of some thousands of Polish members of the army, of the police forces, of the previous government and many landlords and capitalists. If you ask a Pole they'll invariably bring up Katyn massacre (which is generally attributed to the Soviets but AFAIK there's room for doubting that, given some key pieces of evidence such as examination of munitions used in the executions revealing that they were German munitions). The thing is: the Soviets weren't "annexing land" for the sake of annexion, they were propagating socialism, and to have socialism, you need to make sure you won't suffer a capitalist counterrevolution organized by the formerly powerful ones who were benefiting from the exploitation of the workers and peasants.

Overall, I do believe that the expansion of the Soviet socialist model was a good thing in and out of itself, and therefore I generally defend the actions taken to expand this model to countries that resisted because I believe that the ones resisting this weren't primarily the workers or peasants, but the ruling classes and the wealthy.