Having a random applicant "work" a day for free is a pretty crappy proposition in most industries; I suspect tech isn't much different. You gave an example of asking a job applicant for goldman sachs to "trade stocks for a day." Are you kidding? Do you realize how much money a bad applicant could lose in a day? If it's different in tech, it's only because it's (often) easy and painless to roll back an applicant's mistakes.
There are a few ways to approach a job interview. The one I like is to say, "I'm a professional working in this area; those people are professionals working in this area; I should learn what they're up to and we should see if it makes sense for us to work together right now" and right now is key, because sometimes the timing or the particular projects aren't a good match, but the people are and you'll want to stay in touch in the future.
One nice non-coincidental advantage of treating it this way is that "success" is a lot easier to achieve, so the interview's going to be a lot less stressful.
So now approach it from that perspective. You could spend a day (or two hours, or two days, or whatever interview duration) chit chatting, talking to HR people about benefits, describing past projects and whatever. But time is really short and the important questions are: what are you actually going to be working on? and are the people you'll be working with any good? Are their experience, personality, skills, etc. going to set you up to do well, help your career, get stuff out the door, and whatever other concerns you have. How are you possibly going to learn any of that without talking to them at length and in detail about their projects? And the best way to "talk in detail" is to pretend you have the job for a day and get up to speed.
So are you now "working a day for free?" Not really. Because, if the people you're interviewing with are people you'd want to work with, they've had a lot longer than you to think about their problems and know a shitload about them. If you contribute anything of value that day, they probably would have gotten as much value from talking their sticking points over with any bright, engaged outsider to get a fresh perspective.
In the rare occasions where the applicant really does provide high value consulting for free for a day, he or she should have known better than to interview there in the first place. If there really was no way to know in advance, just accept that you had a dud of an interview and go on with your life. You've still learned whether that particular company is one you'd want to have a business relationship with in the future, which is valuable, and whether any of their employees are particularly good or bad, which is also valuable; and, if the company has the right values, there's a good chance that they'd want to hire you for contract work. So even those interviews still tend to work out.
tl;dr - when you interview for a job, no matter what, you're working. You'll get the most out of it as an interviewee if you take it seriously as work and try to get as close to the day-to-day job as you possibly can. If the company materially benefits as well, even better.