Hello lachlan786
In Switzerland, and many other European countries, a train ticket and a train seat reservation are two different things. You can use a Swiss train without a seat reservation, with the exception of a few tourist trains. We, the Swiss, are using our trains like suburban trains in big cities: arrive at the station, what for the upcoming train, hop on, have a ride, hop off, done.
We have a written a comprehensive guide about seats reservations on Swiss trains:
https://community.sbb.ch/blog/23887-demystifying-seat-reservations-on-swiss-trains-a-comprehensive-guide
SBB can sell any train ticket for any Swiss public transit company (railway) as all do use the same ticket vending system.
The seat reservation, however, is the responsibility of each railway company; therefore you have do to the seat reservation with the operting railway; SBB is simply notifiying about the need of such.
Bernina Express has some cars that do not require seat reservation; they are intended for the locals who need the train to go about their daily business. Also regional (R, RE) trains do not need a seat reservation. Yet, RhB has recently published a warning saying that the Bernina line is suffering from mass tourism and travellers without a seat reservation shall expect long waiting queues. Some restrictions were installed to accomodate the masses, and they are most likely to be continued.
https://www.swissinfo.ch/eng/various/overcrowding-prompts-restrictions-to-popular-bernina-train-line/90680906
Kind regards,
Roland