Why was my US visa stamped at immigration?

Stamping the visa - usually on the edge so both the passport and visa are marked - means something to CBP officers, but it means nothing to you. It's a trick of the trade to discourage fraudulent reuse of stolen visas. read more

On entry, the immigration officer will check the validity of the visa stamped into your passport and make a determination whether you are allowed to enter. You may receive an 'entry' stamp as well. If admitted, you'll be issued an I-94 and/or today in most situations the information is scanned in through your passport and you can get a copy electronically. read more

It's impossible, at least given the setup of the vast majority of (if not all) US airports. In these airports, because there is no exit control, it is possible to leave the departure lounge, without boarding your flight, and also without passing through the immigration hall. read more