Tbf you could say that the Navy is lenient to Shanks as well. They had multiple undamaged top tiers in MF, if they really wanted to I think they could have taken him and his crew down while suffering heavy losses.
Sengoku even says "
I'll take responsibility, because it's you, red-hair." which somewhat implies that his response would've been different if it wasn't Shanks.
This attitude makes sense because Shanks is:
1. A CD by blood, even if he did abandon his standing.
2. A powerful Yonko and a very important contributor to world balance
As for pirates, IIRC Shanks only gets ruthless around provocateurs like Kid, and even Barto was messing around in his territory, right?
Plus he's also got a bunch of weaker crewmates whom he has to protect, hence why he went all out against Kid instead of fucking around.