Apparently, serving staff in the US are at minimum wage because the management "assumes" patrons will tip, increasing the serving staff's income by 20%. Then, taxation takes over, assuming that the serving staff are making 20% more so, if this is the case, it shouldn't be shocking but, I could be wrong

.
It's worse than that even. The restaurant actually pays less than minimum wage, because the restaurant is allowed (required) to include tips as part of the employee's taxable income, from which their actual hourly rate for Fair Labor Standards Act (minimum wage/overtime) purposes is calculated. The restaurant offers a base wage so they have a foundation to track the server's payroll (last time I was in the game it was about $2.30/hr) and the server's expected to earn the rest of their income in tips (and theoretically to report them, though servers love cash tips for the lack of paper trail). The restaurant is, in theory, liable for making up the difference if a server's tips don't cover minimum wage for the hours worked in a week, but if that's the case the server typically isn't long for that job.
This is changing, but very slowly. Opposition to changing the status quo comes from many sides, and is closely tied to the larger fight over a higher minimum wage. The biggest single problem is one we already have, that would get much worse before it got better; customers being aware of when and where the person serving them is "tip-dependent". The first restaurant to adopt a fair-wage model in any particular market is going to have a "first-mover disadvantage" inherent in the higher prices on the menu compared to their competition, and competition in the restaurant business is
very cutthroat, with margins extremely slim. It usually takes a massive marketing barrage, like Southwest's "Bags Fly Free", to let customers know what the deal is and why what they're paying is worth it versus their competition. You also have to define "fair-wage"; the US minimum wage puts a single earner just above the Federal poverty line, and a family of three below it. A restaurant paying minimum wage and marketing that as "fair" to the public would likely end up being a pay cut for waitstaff, as patrons would think "not tip-dependent" and tip little or nothing, while the restaurant isn't covering the difference. So, "fair wage" for tip-dependent workers gets entangled in the fight over "minimum wage" for
all workers (service industry and otherwise), which is a hotly-contested debate for a variety of practical, political and theoretical reasons that this post is already too long to include.
The most accepted compromise is for all full-service restaurants to make the gratuity required for all parties. This is already policy at many restaurants for large parties or high total amounts, to prevent the person getting sticker shock and stiffing the waitstaff. The law in most US jurisdictions is that you are liable for the amount on the bottom line of the check unless there is an articulable error in the charges, so adding 15-20% as a base gratuity that the customer is now required to pay means the server gets theirs much more reliably, and the customer can still tip more if the service really went above and beyond.
Like any compromise, nobody really likes it. Customers don't like it because a traditional signal of bad service is a small tip; if the minimum possible tip is 18%, and the servers know that, then the chance of poor service is higher because the pay's the same whether you try or not, and there's little recourse other than the Karen-esque "I need to speak with your manager". Restaurants don't like it because their floor managers spend more time listening to complaints about how the service wasn't worth the minimum tip (or the simple fact that they
dared to
make the customer pay the tip they likely would have added anyway) than they do preventing complaints by keeping things flowing normally. And waitstaff don't like it because patrons who might normally tip 20% or more see an included gratuity of 15-18% and don't add any extra, making this system a pay cut at established high-end places with well-heeled clientele.