This is not true. Requirements for PTC are different in each state. Why should someone from one state who requires no training to get a PTC be allowed to carry in a state that has stringent training requirements? Either the states have authority to place reasonable restrictions on the 2nd Amendment, like SCOTUS has said, or they don't. If the federal government can dictate to the states what those restrictions can be, the 10th Amendment and the SCOTUS decisions are meaningless.
As one who thinks such training and other requirements are unconstitutional, I’m not moved by the depiction of them as “reasonable restrictions” with which noncompliance evokes a suspension of enumerated inalienable rights. They’re not reasonable restrictions.
Driving requirements differ by state too, but a GA license is valid in NY nonetheless.
And, to run with your example anyway, what of those states whose requirements are less than those of other states? Having received legislated training, extensive background checks, serial-number-level registration, and compliance with any other restriction equivalent or more extensive than any other state, why should a citizen be denied RKBA on the sole grounds that the state visited has not issued identical or inferior paperwork?
Read the Constitution. Federal law is supreme and prevails over any state law to the contrary. There is a necessity for uniform federal legislation to make certain that when it comes to the RTC, the least restrictive state standard is applied throughout the nation. This is the way it should be.