Yes, drawing the line is difficult. Law is not mathematics. A judge must apply his judgment to decide whether a particular court order represents an "unreasonable burden". That is the precedent set by the case United States v. New York Telephone Company. An unreasonable burden for a one-man development shop is not necessarily an unreasonable burden for Apple. Judges are fully capable of considering this.
The precedent for United States v. New York Telephone Company has just recently been judged not to apply to Apple unlocking an iPhone. And unreasonable burden is not the only criteria. In addition, this specific situation has First Amendment implications that United States v. New York Telephone Company did not (although this has not been tested in court yet).
