And I’m arguing, rhetorically, what if your name produces a syntax error—or worse means something semantically devious—in the query language I’m using? Not all problems look like script tags and semicolons.

It's a question of intent. There aren't any hard and fast rules, but if someone has chosen their company name specifically in order to cause problems for other people using your service, then it's reasonable to make them change it.

This is getting really absurd. Are you also going to complain that Unicode is too restrictive or are you going to demand being able to use arbitrary bytes as names. Images? If Unicode is enough, then which version.

There is always a somewhat arbitrary restriction. It's not unreasonable to also take other people into account besides the user wanting to enter his special snowflake name.