Actually Historically speaking they are polarized... a polyester film capacitor is two long parallel plates (ribbons but the idea is the same) and the polarity came from old tube days and to a point even today for capacitors of similar construction. The major exception being metalized layered monolythic caps.
The standard american symbol looks like this: This end was considered the inside foil and therefore more susceptible to noise pickup > -----|(----- < and this end was commonly accepted as the 'ground' end or outside foil. "From the 1949 Radio Amateur Radio Handbook" Published by the ARRL.
{Edit RKJ}
Bob