>When in Rome you do as the Romans.

Try something besides this, which amounts to "because I/they/we said so".

>If you know that doing something arbitrary might offend your
>host or make you look like a bama, and if it's not a big deal
>to you and doesn't cost you anything, then what's the point of
>not doing it?

It's no big deal in practice, you're correct. Which makes your insistence that people placate the need to continue this stupid custom that much more petty because the inverse is equally true.

>It's kind of like why I wouldn't wear a brown suit to an
>interview. It's not a big deal to me and I know that a lot of
>people think that only blue or grey suits should be worn to
>interviews (the type of interview that would require a suit of
>course) and wearing anything else they would consider tacky,
>why would I wear a brown suit?

It's nothing like that, unless you're going to an interview. You're going to that interview trying to get something with the obvious implication of a need to impress the person you're trying to get something from.

So yes, things like that are often arbitrary but there's a specific goal. You go to work and make concessions in exchange for the benefits of your job up to and including that pay check and similar concessions are made to conform to what you hope are the standards of the people interviewing you for that job.