Every palm tree in local landscapes is exotic. Simply put, all palms are originally from somewhere else. The desert fan palm, the only palm that is native to California, came from isolated colonies in desert regions many miles away. In fact, most plants in common landscapes are exotic. Landscapes composed of Californian plants likely include some plants from other regions of California.
With few exceptions, exotic plants are not a problem. However, some of those few exceptions have become very serious problems. Himalayan blackberry, blue gum, silver wattle, pampas grass, giant reed and broom are some of the more notorious examples. They naturalized to become prolific and aggressively invasive weeds. Some are more common than natives in many situations.
Naturalized exotic plants such as these are problems for local ecosystems, even if they do not affect refined landscapes. They compete with native plant species for limited resources, space and pollinators. A lack of pathogens from their homelands can be a distinct advantage. They alter the lifestyles of some of the native fauna. Some enhance the combustibility of the forests they inhabit.
The justifications for importing exotic species are as varied as the species themselves are. It might have been for lumber, forage, fruit, or vegetable production. Giant reed might have arrived here as packing material for cargo from southern Asia. Nonetheless, most naturalized exotic species, including the most aggressively invasive, came here simply for home gardening and landscaping.
Realistically, of all the countless exotic species that came here during the past few centuries, very few naturalized. Fewer are now aggressively invasive. Some with potential to naturalize may not have yet been able to escape the urban situations they inhabit. The problem now is that there are so many more exotic species readily available from all over the World than there has ever been!
Online marketing facilitates procurement of exotic and potentially invasive plant species from other regions, with minimal regard to regulation of such commodities.