How come snails won't cross copper tape?

Answer 1

They wont cross for the same reason we use copper in electrical wires. It has high conductivity meaning electrical current passes through readily.
Snails are 'wet' and have an electric potential stored in their cells. If they try to cross the copper the current path to neutral ground is completed and they discharge a bit on contact. This is not enough to kill but is an unpleasant sensation able to discourage snails.
The tape does not generate a charge it only allows a current to move from high potential to lower.

Membrane electric potential and ionic environment
http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mcb.section.4055
"An inside-negative electric potential (voltage) of 50 – 70 mV exists across the plasma membrane of all cells; this is equivalent to a voltage gradient of 200,000 volts per centimeter."

Answer 2

It shocks them literally. Their slimy body contacting the copper is like completing a circuit.