Dust and small particles in microgravity environments are generally regarded as bad, and items prone to generating these tend to be discouraged:
Bread should be prepared quite differently, so that it does not crumble and lasts a long time. The Russian Institute of the Bakery Industry had to come up with solution for using the bread at the International Space Station without the risk of crumbs getting into the ventilation system. They came up with a miniature loaf - just big enough for one bite. In texture and taste, this bread is virtually indistinguishable from that which we eat on Earth. ["How Russian space food has evolved over the years" by Russia Beyond, found via this answer to the question "Why was there a miniature bread on the ISS?"]
Bread: Astronauts are not allowed to eat bread because their crumbs can go into machine and equipment, and into astronauts' eyes.
Salt and Pepper: Again similar to bread, salt and pepper can go into their eyes and damage equipment. Salt and Pepper are served on the ISS by being dissolved into liquids. However salt and pepper in their raw form are not allowed on the ISS. [This answer to the question "Are there foods that astronauts are explicitly never allowed to eat?"]
The biggest issue is that chalk notoriously produces dust, which will not settle and becomes a breathing hazard.
Pencils and grease pencils were phased out as they created particles and shavings, but at first they didn't have anything better.
The writing with the chalk will work GREAT! The astronauts and spaceship that has to deal with the liberated chalk dust will NOT do so well. [This, this, and this comment on the question "Can we write with chalk on blackboard in space?"]
The stated reasons have to do with the risk of floating crumbs or particles getting into machinery, lungs, or eyes due to their not being pulled to the floor by gravity.
- Proper air circulation (more important in microgravity than in macrogravity to begin with, due to the need to prevent pockets of carbon dioxide from building up and asphyxiating astronauts) and filtration should effectively clear floating particles from the air;
- Anything floating into an astronaut's eye would trigger the blink reflex and thereby be cleared;1
- Electrical and electronic machinery tends to be quite insensitive to electrically-nonconductive dust until so much of it has accumulated for it to impede equipment cooling by its sheer airflow-blocking bulk (something we know because electronics are dust magnets even in macrogravity, as anyone who's ever opened up a well-used desktop knows), and equipment with conformally-coated circuit boards will ignore even electrically-conductive dust (e.g., graphite from pencil shavings), and will shrug off liquids to boot.
So why is dust so feared in microgravity?
(Thanx to DrSheldon for prompting me to ask this question.)
1: In fact, eye-attacking particles would probably be less dangerous in microgravity, where they'd just be floating around, than in macrogravity, where particles that get into eyes tend to have been thrown with considerable force from whatever generated them.