Why is freezing bacteria not a problem?
The ice crystals that form tear and shear the bacterial cells, unless you add a cryoprotectant (for example, glycerol) to the cells beforehand.

Remember, when water freezes its volume increases.  This is why a closed soft drink can that you put into your freezer to chill faster, will burst if you leave it in until the contents are frozen.

Exactly the same thing happens to cells (animal cells, plant cells, yeast cells, etc.) if they are frozen without a cryoprotectant.