Some ceramics heat up in the microwave. You have experienced this probably, whenever the bowl is searing hot but the food cold. That's when a thing is not microwave safe.

When it's microwave safe it won't absorb the waves and won't heat up.

how? HOW CAN A MUG NOT BE MICROWAVE SAFE? please tell it isnt cheap plastic

talking pikachu mug with a speaker and light up tail

does this exist?

The starbucks in my area used to sell the mugs that they also used in-store. They were gigantic (like 700ml?), completely white ceramic with a very subtle logo and only cost 10€ each. They're also extremely dishwasher proof because they were made to be washed like a dozen times a day. After like 6 or 7 years of use (we got them pre-covid), they still look like new and I regret only buying 2 of them at the time.

Ironically, I haven't actually ever bought a drink at starbucks. I only bought the mugs because I saw them while picking up a Too Good To Go order there one time

I've made that mistake a couple times. I've just washed the mug in the dishwasher until all the prints peeled off and it became a normal mug.

Some people try to buy a personality

The big dishwasher-unsafe novelty mugs are good for keeping on your desk to hold pens or mints or something.

Somebody wants to drink their coffee out of Marvin The Martian and this poster is committed to not let anyone enjoy such a thing.

why hot wash?

I wash everything cold, and generally all together

I have two starwars mugs with all the light sabers from differentcharacters around it, so when the hot water goes in they light up. It's great.

I only have two because my wife got one and I put it through the dishwasher and ruined it. So I bought two more

I have some hand made mugs and the bottom of them are raw(?) undglazed(?) which is pretty bad for them. I still do it, and so far they are still alive

Some of those with printing on them would fade pretty badly.

Generally speaking, we should not be eating hot food from plastic containers. I certainly don't.

Ideally, we wouldn't use plastic containers for food at all, but that ship sailed 60 years ago. Also, even if you are able to avoid buying food in plastic containers, the machines used to process the food, even just to transport fresh vegetables along a conveyor line, shed plastic from gears and bushings, etc. But, that's micro plastic, not the chemicals that leech from the plastic much more readily when it's heated.

Here's a study about micro plastic in olive oil. TL;DR: there's no difference in the amount of micro plastic in olive oil between oil sold in glass containers compared to oil sold in plastic containers.

Kitchen knifes are also infamously not dishwasher safe. Mine's got wooden handles but they were slightly worn from not drying them immediately when handwashing. Ever since I got a dishwasher I chugged them in there and they have survived without any more damage.

Surely you can say this about practically everything you put in a dishwasher.

Dishwashers aren't really meant to be quick. They're meant to be efficient and set/forget.