shabbat - Why is asking a non-Jew to turn on electric lights considered amira l'nochri?

Given that practically all poskim accept R' Shlomo Zalmen's view that electricity is d'rabbanan over the Chazon ish, I would think that a Jew could simply ask a non-Jew to turn on/off lights on Shabbat since it should be sh'vus d'shvus. However, common practice where I live is to avoid asking or even halachic remiza to the non-Jew. Halacha classes and pamphlets by local rabbanim deal with the issue based on the premise that it is assur to ask.

Question: What is the reason to forbid just asking?

For the purposes of this question we will assume that the light in question is a CFL to avoid the side issue of incandescent bulbs.

Answer

As was pointed out by @DoubleAA in the comments, the turning on of many types lights is biblically prohibited on the sabbath. I will address in this answer those types of lights assumed by some to be only rabbinically prohibited.

Even something which is only rabbinically prohibited is included in the prohibition of Amira LiNachri (asking a gentile). The only allowance to ask a gentile to do a rabbinically prohibited action is a case of need, (or great need, according to some authorities).

The Shulchan Aruch 307:5 states:

ה. דבר שאינו מלאכה ואינו אסור לעשותו בשבת אלא משום שבות מותר לישראל לומר לאינו יהודי לעשותו בשבת והוא שיהיה שם מקצת חולי או יהיה צריך לדבר צורך הרבה או מפני מצוה כיצד אומר ישראל לאינו יהודי בשבת לעלות באילן להביא שופר לתקוע תקיעת מצוה או להביא מים דרך חצר שלא עירבו לרחוץ בו המצטער ויש אוסרין. הגה: ולקמן סימן תקפ''ו פסק להתיר ועיין לעיל סימן רע''ו דיש מקילין אפילו במלאכה דאורייתא ועיין שם סעיף ג':

My loose translation: Something which does not constitute [Biblical] "work" and is only prohibited on the sabbath by the rabbis one is allowed to ask a gentile to do on the sabbath, provided that there is some minor illness [which necessitates it] or if there is a great need, or for a mitzvah... And some prohibit [even in those cases]. The Rama says, and later...[The Mechaber] holds like those who permit. And see 276 that there are those who permit even a biblical prohibition.

As you can see, it is clear that all agree that without at least some need, we do not permit one to ask a gentile to violate a rabbinic prohibition.

As for hinting, there is a common misconception that one may hint to a gentile to do prohibitions on the sabbath. While there is some basis for this (see end of OC 307 and MB there 76), it is certainly prohibited where the Jew receives what halachically qualifies as benefit. What falls into this category is not clear, but lighting a dark room certainly does, see OC 276 at length.

That is why people refrain from asking, or even hinting to Gentiles to turn on lights. The question is if there is any rational for permitting.

(If there is already a source of light in the room there are many authorities who would permit one to hint to a gentile to turn in additional light. However, this is far from obvious).