The pattern identified in the question is generic in American criminal statutes, and contrary to the "set and forget" theory, it continues to be true even in states that have recently amended the dollar amounts in their criminal codes.
Also, usually, the relevant statute states for all but the most serious offenses, that a court may impose on a defendant the maximum authorized incarceration sentences, the maximum fine, or both. So, frequently, in the case of a moderately serious offense, it isn't an either/or choice, and instead the fine is imposed in addition to the incarceration sentence if one is imposed.
Incidentally, this is not true globally.
Some jurisdictions, particularly in Scandinavia and Northern Europe utilize the concept of a "day-fine" in which a fine is set at a level comparable to one day of income for the convicted defendant who continues to live in the community working in order to pay it. In those countries, day-fines are the normal punishment for minor offenses (e.g. speeding or shoplifting, for example) and for what would be low level felonies in the United States. Incarceration is then reserved largely for very serious offenses (with terms of incarceration that generally are much shorter than those in the U.S. for comparable offenses) and for defendants who represent a clear and present threat to the public if allowed to be at large (basically violent offenders with weak ties to the community).
In countries with a day-fine system, there is an either/or choice and days of incarceration and day-fines are exchanged for each other one to one from within the sentence authorized upon conviction for an offense.
The motivation for the U.S. pattern of fines has other factors behind it.
One is that for a large range of offenses, convicted criminal defendants in the U.S. usually don't have much of an ability to pay after meeting their bare minimum of daily needs for food and shelter (and not infrequently are homeless and/or unemployed). So, imposing large fines often gives rise to futile and wasteful government entity fine collection efforts, may increase recidivism by creating a need for money for essentials that the convicted defendant can't pay since his or her money went to fines, and can reduce the funds available for restitution or a civil judgment that actually benefits the victim related to the same course of conduct.
Second, for the vast majority of misdemeanor offenses where there is both a potential term of incarceration, jail sentences, when imposed are typically far, far shorter than the maximum sentence.
For example, it would be common in many U.S. jurisdiction for the normal sentence for a misdemeanor or ordinance violation to be a fine and time served over a weekend awaiting a hearing before pleading guilty (plus restitution and court costs and/or community service), and for a sentence of incarceration imposed when someone does not plead guilty and/or the gravity of this particular offense is particularly serious or there is especially great community outrage, to be a week to 30 to 90 days for an offense for which a sentence of 1 year in jail is authorized by law.
Limited jurisdiction court sentencing judges who handle misdemeanor and ordinance cases have immense sentencing discretion which they rarely utilized to the fullest.
Effectively, what legislators are doing in this kind of system is saying that they are bothers to figure out what sentence is actually fair for a host of minor offenses and are delegating the authority to make that decision to judges in the form of routine custom and practice in high volume courts, and are giving parties an incentive to cooperate with judges rather than bogging up the system by being difficult without exceptionally good cause for doing so.
Likewise, in the case of minor felonies with maximum incarceration sentences authorized of three years, two years, or one year, for example, a majority of criminal convictions in U.S. state courts (at least when there is a plea bargain to the offense) are disposed of with a probation sentence plus fine, restitution and court costs, or with an actual sentence of incarceration of 30 days to six months or so, to be actually served in a local jail rather than in a state prison, rather than anything approaching the maximum sentence authorized by law.
When the amount of the fine for an offense level is viewed in relation to a typical sentence actually imposed for that offense after a plea bargain and with no notable aggravating factors, the fines are typically much more proportional to the typical incarceration sentence than they are to the maximum prison sentence, which is often only really imposed with full force on defendants who go to trial in cases where there are also exceptional aggravating circumstances.
It also isn't uncommon for a state to have a separate set of fines that are significantly higher for corporations that are convicted of crimes, which represents what a fine would be if it was the exclusive punishment that a court had available to it to impose for a crime.
