I tip 20% at restaurants. I don't get the tipping culture. If you get bad service and don't tip I can only imagine the insults that would get thrown your way. Knowing people in the industry regarding restaurants and big bars it's not uncommon for the bar tenders and waiters are forced to pool their tips at the end of the day. The manager takes it all to his office and divides up the money taking a huge chunk for himself.
Regarding a barber I give a $5 dollar tip. I know many go to beauty parlors that charge a lot but I just go to a barber. I get my hair cut every third week.
Are you aware that tip pooling is addressed by the U.S. Department of Labor? https://www.dol.gov/whd/regs/compliance/whdfs15.pdf
Here is another good read on the subject of tip pooling: https://www.nolo.com/legal-encyclopedia/tip-pooling-credits-service-employees-29804.html
Tips are treated as income by the IRS. As such there are tax laws pertaining to tips. https://www.irs.gov/businesses/small-businesses-self-employed/reporting-tip-income-restaurant-tax-tips
Without doubt, there are employers who don't comply with these rules.
In Germany, where I visited this fall, the norm is to round-up the bill as a tip. If there isn't much to round up, folks sometimes add a Euro or so as a tip. Service employees earn a decent wage, usually with benefits, such as sick leave, health care insurance and vacation. Americans who aren't aware of this sometimes leave tips like they would in the U.S. a rule of thumb is to tip between 5 and 10%.