Why shouldn't a precooked pie be frozen?
Generally, these warnings are for food quality reasons, not for safety reasons. 

Assuming this is a fresh strawberry pie, I'd guess that - because strawberries don't bake or freeze particularly well - if you freeze the pie, the fresh strawberries will turn to mush and not taste very good, particularly if you try to bake it to heat it up.

Freezing something should never make it unsafe to consume... freezing is a preserving method.