I'm not sure though what Finland could have done otherwise; if they had not been where they were, Leningrad would have received shelling from all sides, and like you say, the city would have been under siege anyway - possibly with worse consequences, since the Lake Ladoga route would've likely been cut then as well.

Actively resisting the Germans in their aspirations was not feasible, as Finland needed the German military muscle to repel Soviets then and in the years to come.

So, Finland could not not be there, and if Finland had not been passive, then the only option available would have been to actively contribute to the siege and fire at an innocent civilian population. I'm not saying the decision to avoid this was because of moral or ethical concerns; the reason was probably more mundanely a lack of material. Artillery shells and bullets were best reserved for soldiers and not starving civilians in a strategically uninteresting city.

A side note about the "Nazi alignment": if that taints Finns, then it should, strangely enough, taint Russians as well; even the Soviets aligned with Nazi Germany. The secret clause of the Molotov-Ribbentrop pact divided Europe to Soviet and German spheres of influence. Then, soon after the pact was signed, Poland took the first hit in 1939 with attacks by both Nazi Germany and Soviet Russia. And weirdly enough there was even a joint Nazi-Soviet military parade in Brest-Litovsk.