What does it mean when someone is suspected of illegal activities on SW?

It really depends but they did something against the sites TOS.

Con games ... financial fraud ... pedos contacting minors not permitted under state law ... stalking inconsistent with restraining orders ... communications by fugitives ... and the best ... online Harassment, as defined by NY, CA and MD law

I wondered that too

Selling weeds and talking about vandalizing property and people to get money for it. One guy confessed that.

the guidelines and legal pages should tell you what is considered suspicious and illegal.

Sex trafficking.

Unsolicited dik pics

FYI.. they love the dik piks ..

Sodomizing beagles

Maybe farted