Why? Those two are the same to my ESL sensibilities: one Taco Bell, two Taco Bells; one Mountain Dew, two Mountain Dews. "Bell" may be a name of a specific person, but that's arguably subsumed by the "Taco Bell" being the name of a company. That may be a proper name in context[0] of all local companies, but once you use it to refer to restaurants - as in, "Taco Bell restaurant" with "restaurant" implied, then why wouldn't plural be "Taco Bells"? One ${opaque name}, two ${opaque name}s, etc.
Same with Mountain Dew - one Mountain Dew[ drink], two Mountain Dew[ drink]s...
EDIT:
The answer here seems to agree with my thinking: https://english.stackexchange.com/questions/190023/why-are-n....
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[0] - There are no truly unique names; there are only names referring to a specific entity in the context they're being used in.