Legal Question in Construction Law in California

Probably. The utility poles and wires are undoubtedly on a valid easement, and the general terms of utility easements require the easement holder to maintain its own facilities within the easement, but also a a general matter the owner of the property upon which the easement lies must not do anything that intereferes with the easement holder's use of the easement for the intended purposes thereof. Growing big tall palm trees (probably now taller than when the utility lines were designed and strung) could certainly be considered an interference with the easement's use. Tree owners are generally responsible for damage done to others' property, and I'd think that would be true where the property is on an easement. Further, the utility is unlikely to make a threat of this kind unless it was pretty sure it was consistent with court decisions. It's entirely possible that the original easement agreement may have specific language to this effect.