I just noticed this question being left unanswered. I will try to keep my answer short and unfortunately sources are in Finnish, so further research on your part might be difficult.

After the Moscow Armistice on 19 Sep 1944 USSR demanded driving out the Germans in Lapland, banning of fascist (and 'fascist') organizations, legalizing the communist party, war reparations and setting up an Allied Control Commission to mention a few. Finland started drifting towards pro-Soviet sentiment in the following decades. This was enforced by political pressure and gaining control of the State Police (ValPo, secret police) amongst other things.

The overall amosphere was headed towards the idea that Finland was the aggressor and the Republic of Finland should do anything in its power to appease Soviet Union for past wrongs. This process is called Finlandization. While USSR was strong, its influence on Finland was strong as well.

One result of this was that Finnish war veterans, Simo Häyhä included, were labeled as war-mongerers, anti-bolsheviks, fascists etc. The sentiment was that veterans had been voluntarily siding with fascists and opposing the Great Patriotic War as it was called in Soviet Union. Only after USSR's collapse has their reputation been restored and they have been openly celebrated again.

Some historians claim that the general opinion post 1945 was not as negative as it's often remembered, but a vocal, powerful minority called 'taistolaiset' were very loud with their critique and rest of the political factions were too afraid of USSR to oppose them out in the open. So it became the only heard voice and brought up a myth of a 'consensus of loathing' towards veterans.

Feel free to ask any follow-ups.

• Korhonen Sirpa: Veteraanien sotakokemukset ja hyvinvointi: Seurantatutkimus pohjoissavolaisista sodan kokeneista miehistä vuosina 1993 ja 2006. Publications of the University of Eastern Finland. Dissertations in Social Sciences and Business Studies, 2011. ISBN 978-952-61-0380-8.
• Juujärvi Soile, Pesso Kaija & Salin Ossi: Sotiemme veteraanien identiteetti ja arvostus. Helsinki: Edita Prima, 2009. ISBN 978-951-799-157-5.
• Jokisipilä Markku (toim.): Sodan totuudet: Yksi suomalainen vastaa 5.7 ryssää. Helsinki: Ajatus, 2007. ISBN 978-951-20-7533-1.
• Kivimäki Ville: Murtuneet mielet: Taistelu suomalaissotilaiden hermoista 1939–1945. Helsinki: WSOY, 2013. ISBN 978-951-0-37466-5.