Q. Why am I required to pay so much to the Comptroller of Maryland when I don't have a business in MD?
A. Because you are a full year resident of MD. This rule applies in all states*.
If PA also taxes your income, MD will give you a credit, or partial credit, for the state tax you paid to PA.
* This is the general rule: The income is work state (WS) source income since it was earned there. Resident States (RS) tax all their resident's income, regardless of where earned. You will file a non-resident tax return for the WS and report the WS income. You will file a full year resident return for the RS, reporting all your income. The RS will give you a credit, or partial credit for any tax paid to the WS.
For state filing requirements, the WS does not, usually, go by what you earned in their state but by your total income .
Filing Requirements: https://ttlc.intuit.com/questions/2903200-do-i-need-to-file-a-state-return
