I think the reason why people would be pro-draco but anti-snape is for a few reasons.
Raising your own kid to be hateful and bigoted can be considered it's own form of abuse(something similar happened to Dudley. The point where this stops being abuse is when they're old enough/have the opportunity to gain perspective. Draco, Dudley, and Severus all get that change in perspective)
Reader's/watcher's can more clearly tell the driving force behind Draco's words and actions because it gets brought up all the time(up until book 5, reader's don't get anything except hindsight for Severus' story)
While Severus did change sides and redeem himself, he was a dick to most of his students and made it very clear that fairness wasn't a high priority(I understand that this was to uphold his reputation so he could continue to be a spy, and to try and counteract the Slytherin bias, but 2 wrongs still don't make a right or brownie points
TL;DR: Raising a kid to be a bigot is abuse, let's not play into trauma olympics. Severus' abuse and trauma didn't get enough page time Severus continued to be a dick post-redemption. Draco had his redemption at the tail-end of the series and didn't get time to continue being an ass or not
Do the “Draco was just a boy, Snape was an adult!” crowd realize that Severus is significantly younger in the books? Severus was only 20-21 when he turncoaked The Deatheaters and become a spy, the brain doesn’t develop impulse control and non-b&w thinking until 25 at the absolute earliest and prolonged trauma (which Severus had and Draco didn’t) physically damages the brain, slowing that process down.
Severus was groomed as a teenager (about the same age as Draco was) into a rising cult by an, at the time, charismatic young Tom Riddle who was at his height of manipulation and power. And Severus was canonically abused by a muggle man and by Gryffindors mercilessly his whole developing years. That crowd incessantly fanons that Draco was “abused” by his parents when he wasn’t but hate Severus like make it make sense.