With net neutrality gone, an ISP absolutely could do that.

Even if you could, does that make it right that big tech companies are censoring people and organizations for their views?

Social media platforms and telcos operate at different layers of the stack. Even if something's acceptable at one layer, doesn't mean it's OK at another, because the consequences are different.

If an ISP allows someone to express something offensive (i.e. legal, but strongly disagreeable to the majority) on their network, it doesn't affect their other customers directly - they won't know or care which ISP this person was using. If a social media platform allows posting something that most other users would find offensive, those other users will leave. And yet people are arguing for deregulating the first (anti-net-neutrality) and regulating the second. It seems backward to me.

What you call "censoring" someone else might call "curation and removal of BS". The free market can decide whether it's too much or too little.