I don’t.  Most people I work with wouldn’t tolerate that from labor, either.  You work for what you agree to work for.  You don’t ask for more.
Davis-Bacon only applies to public works projects.  For private projects, including work on residences installing something like a patio, it is entirely legal and entirely common for the labor to get less than Davis-Bacon.  Why do they do it?  Because if they don’t they next guy will work for less and the first guy will have NO work.  If you have a choice at working for $20/hour and getting NO jobs and working for $10/hour and getting something you’ll pick the latter if you have bills to pay and a family to feed.  Basic economics, think about it.
And if there are problems or overruns it comes out of those profits.  The general contractor I work with most often (in part because he is extremely reliable at paying his labor - not a universal trait, unfortunately, these days) has worked jobs that actually COST him money due to problems coming up.  If there’s a screw up it comes out of the boss’s pocket first.
Yes, when things go well he makes a profit.  But he does run a real risk that if the crap hits the fan he’ll lose money.
But a lot more are catching on these days.
Normal care and diligence - which should be to a high standard - doesn’t call for extra BUT if they truly did exceptional work then there is nothing wrong with acknowledging that either verbally or materially.
Which is entirely appropriate - they’re there to work and do a good job.  As they are getting paid for it, and are presumably professional, they SHOULD be busting their butts and delivering what was promised.
But, again if they did more than customary there is nothing wrong with giving them a “bonus”.  Especially in these times.  Or do you think contractors don’t talk to each other?  We know which people are cheapskates and hell to work for, and which ones actually appreciate us.  It doesn’t have to be anything like money - it’s a hell of a lot easier to work for people who make sure I have something cold to drink on a 90 degree day or let me use their toilet when I’m spending 10 hours a day at their house working on the exterior.  Funny concept, we’re people, too.  It makes it a lot easier to do work that can be hard labor and filthy when we know we’re appreciated.  And we’re more inclined to go the extra yard for the kind clients as opposed to the ones who think their job is to ride our butts.
Fair enough.  I agree, tipping should be optional.  Should you ever hire me I’ll never even hint at a tip - just pay me in full and on time and I’ll be happy.  But don’t get your panties in a twist at the notion someone else might want to give me a “bonus”.  In fact, I stand to collect on a “bonus” of a steak dinner one of my customers gave me at a local steakhouse in appreciation of several jobs I completed for him.   Completely unsolicited - and the same series of jobs has lead to potentially two more customers, which frankly I find far more valuable than the dinner.