Sonic.net

I download Linux distros quite often. In such cases torrents are not only not illegal but often encouraged. Is there a reason why I should not left the client running seeding on sonic network (e.g. it causes problem for sonic for whatever reason)?

"linux distros"

No. You're free to do what you like with your internet service, including seeding torrents or running servers. That's kind of the whole point.

I've had no problem seeding Linux distros over Tor using Sonic, other than the bandwidth will sometimes max out my connection and my connection would drop. I've been doing this occasionally for 10 years.