That's not true. A corporation can be registered just about anywhere within the States and be valid. All they would need in PA is occupancy permits in the name of the corporate entity.
You're confusing "registered" with "incorporated". Yes, they can incorporate in any state, but before they start doing business in another state, they are a foreign corporation and need to register with the state. This is especially important for a state with a sales tax, like PA. I'm sure the city they are doing business in also requires them to obtain a business license.
Feel free to read over some of the legalese:
http://www.legis.state.pa.us/cfdocs/legis/LI/consCheck.cfm?txtType=HTM&ttl=15&div=0&chpt=41A foreign business corporation, before doing business in this Commonwealth, shall procure a certificate of authority to do so from the Department of State
Even worse, AMT is doing business under someone else's name.
https://www.corporations.state.pa.us/corp/soskb/Corp.asp?868855