How a law firm associate can avoid higher taxes as compared to someone in another corporate profession like engineering? Is it because of the fact that law firm treats its associates as retainers instead of employees?

Easy -

1. Accept lower pay.

2. Don't keep lamenting on LinkedIn about no / low stipend (interns), low pay (first years getting paid to learn the ropes).

:P

Join litigation. The pay will be lower than the minimum income required to pay tax.