So if you specify rte=I-90, you not only get all of the regional segments of mainline I-90 from usai, but you all get each of the business loops and spurs in usaib. That seems like a reasonable feature and would not be hard to continue to provide.
That's useful. rte=A1 and similar isn't so much.
The I-90 business routes are related to I-90.
A1 gives you 41 different actual A1 routes (some consisting of several segments and related routes themselves): Albania, Australia, Austria, Belgium, Botswana, Bulgaria, Bosnia, Croatia, Cyprus, France, Gabon, Germany, Great Britain, Greece, Isle of Man, Italy, Jamaica, Jersey, Kazakhstan, Kenya, Latvia, Lesotho, Lithuania, Luxembourg, Martinique, Morocco, Mauritius, Namibia, Netherlands, Northern Ireland, North Macedonia, Poland, Portugal, Romania, Senegal, Serbia, Slovenia, Spain, Sri Lanka, Switzerland and Tunisia. And also two that aren't: Florida's FLA1A and Massachusetts' MA1A.