DARVO

Because other than obvious hate-speech, leftists are the ones who actually practice freedom of speech while the right screams and yells and bans books.

You can get banned here for similar reasons as you'd get banned from things on Reddit. In theory the federated setup helps to mitigate it somewhat, in that if you get banned from your primary instance you can hop over to one that's a bit more agreeable to your perspective and continue on.

For example, I was recently banned from LGBTQ+ on, I think the world server, cause I posted a fairly benign straight opinion to a post that had an image basically asking for cis commentary. It had like 5-7 upvotes, about 13-15 down votes at the time the mods kicked me out -- so even amongst the community it was a bit wishy washy, but the mods still opted to take action. On Reddit, that might've gotten me flagged / banned site-wide, depending on which White House narrative the company is marching to on that day. Here I just lose access to the LGBTQ+ community, shrug at them echo chambering up, and continue about my day.

In terms of "Why do we only control the speech of leftists", I imagine it's because the threads you continue to access are left leaning -- meaning those left in your bubbles, are the left-leaning sorts saying they'd been banned. Right leaning comments, and even (in my view) some centrist / neutral comments, still get people banned. These days we all basically have to assume that there are companies / algorithms creating bubbles in online spaces; you need to temper it with a good bit of secondary sources outside of 'social media' to get a more accurate picture of people's mindsets/trends. Eg. Social Media + direct views of national/local news paper sites + in person discussion with various sorts.