It's the way that law is structured in many states, if not all, actually. In some states (and this includes CA), employers are actually entitled to withhold pay to exempt employees under certain circumstances (e.g. with respect to partial-day absences under appropriately structured PTO/vacation plans, which are trivial for companies to comply with).

That aside, the structure of the law and the practice of companies that are supposed to abide by it don't necessarily coincide. Generally even in labor-friendly states employers hold almost all the power and are unlikely to face any serious threat (e.g. via a lawsuit) from employees who challenge illegal labor practices like failing to pay for a full week in which any work is performed. They're especially unlikely to do so in the software industry, which as far as I can tell has a surplus of labor supply who: a) view themselves as Jon Galt or b) are all too eager to "prove" how great they are compared to other "devs" by, e.g., submitting themselves to onerous employment practices or c) believe, wrongly, that the entire industry is based on merit (intellectual, ability-based merit) and that "unfair" labor practices just can't happen to them (because they're so awesome) or d) some combination of all of that. These things have all conspired to help employers depress wages and maintain the completely farcical myth that "getting paid less for doing what you love" is fair.