Not a business - so everyone who does a one-off service gets out of self-employment tax? 

The issue is was it for services rendered, not whether it's a business.

If I receive a commission/referral fee from my realtor for referring a client, a commission is a Sch C item but I'm not in the business of referring clients so it's other income not subject to SE tax.

For $1,400 at probably minimum wage this person worked ~ 100 hours. That's self-employment income.