I had tried that before using Mathematica instead of a website and I was also surprised to see the data in the barcode is not really signed in any way. IIRC it was a European low cost like Ryan air, and I thought it was scary that I could have generated the same exact bar code just by knowing my name and the flight.
Of course there is an extra step of validation, because the airline has the passenger list, so you can't just add yourself to a flight.
The boarding pass format spec linked in the article [1] shows the support for signing (page 49, fields 25 onwards)
The boarding pass data is still plaintext as explained, but a signature is appended to validate that the content has not been tempered with, and who generated it.
I think boarding pass signing is mandatory on all U.S. airlines at least but I have no source for that
Of course there is an extra step of validation, because the airline has the passenger list, so you can't just add yourself to a flight.