The following is a list of Cisco's trademarks and registered trademarks in the United States and certain other countries. Please note, however, that this listing is not all-inclusive and the absence of any mark from this list does not mean that it is not a Cisco trademark.
Trademarks are exactly how rules for naming things works.
I looked into proton for myself and my partner earlier this year/last year and it was an option. But I think it was 19.99. I don't think they listed it their other plans tho, it was buried in their site. But it was there