the issue is they are under federal law required to protect the POTUS and family members. So it’s not as simple as saying “Welp, we have no dollars left…you’re on your own.”
Additionally, as much as many people can joke about if this particular POTUS were to meet an untimely end that we’d all be better off…the reality is we do not want a precedent set that a funding snafu could leave the POTUS without protection. Also his family members, again despite what you think of them personally/politically, should not become targets because of their relation.