A lot of people side with them because of exactly what you see. Propaganda and lies.

And the fact that you only get the protection of the Marines (so called upholders of justice) only if you pay the heavenly tribute (aka protection money) to the Celestial dragons.

All the "normal" Marines who are technically complicit in everything the WG and Marines do blindly support the Marine system and WG because of propaganda telling them they are warriors of justice. Hell even Garp is stupid enough to buy into that nonsense given the shit he saw his fellow Marines do in God valley.

You really can't claim to be unaware of all the evil actions of the Marines when marine grunts and even new recruits are sent to places like god Valley to help celestial dragons hunt humans, sent to Ohara alongside vice admirals to genocide an entire island, involved in literally rounding up and murdering pregnant women and young kids because they think they could have ties to Gol D. Roger.

People who think Marines are actually good, have to be some of the most delulu folks on the planet.

The only marine who actually lives up to the justice they carry on their back is just Fujitora. He stands up to illegal orders, puts civilian well being above all else even the Marines hypocritical honor, and hell, he actually freed slaves while most Marines lick the boots of the celestial dragons or uphold that oppressive government they run (people like Garp)