They just don't see the rope. They are dialing up your 1-800 number, demanding a translation from your overworked technical support staff, and making your numbers look lousy. All forms of interaction rely on conventions . Matters that are jointly understood by all involved. These conventions can be thought of as the good manners of the interaction, the etiquette of the meeting.
Writing is one kind of interaction, one kind of meeting between parties.
And so, there are also conventions to writing. Conventions that make people comfortable. Make people feel that they know what is going on when they read what you have written. Black text on a white background is a convention. Margins are a convention.