Since it is near impossible to determine if a visitor is a California resident or not, sites/apps will just implement the necessary notices and features to comply with DNT for everyone.
Of course, this is potentially very expensive, since you could have to deal with up to 51 legal regimes (50 states + federal court, if we omit places like Puerto Rico and so on). That's why firms often aim for compliance with the strictest regulatory regime in widespread use, so as to minimize the legal risks. As pointed out elsewhere in the thread, this is why the CA template is likely to eventually become law in other states. Hence the phrase 'as California goes, so goes the nation' - CA policy often ends up as national policy 20-30 years down the line.