Why is this pump not approved for swimming pools?

If I had to guess, it would just be the sheer volume of water a swimming pool has. This is just a small utility pump for smaller jobs. A large pump is what you need for a swimming pool.

It's not meant to be used as a filtration pump. You can pump out a pool. Put he pump in a bucket so the liner will not suck into the inflow area, it could cut off suction.