2. Your partially correct there. The small packets sms uses are pretty light compared to voice or data usage. However, these "sms packets" are actually using the dead space in control packets. These packets, if sent to often, can negatively impact base station switch performance. This is why your texts will go through when you can't make a data or voice connection as well. If the tower gets overloaded with sms, it can crash.
So, the charges are to keep people (and software, which has crashed towers) from over-using it. Once the carriers move to LTE much of this problem will be mitigated, as LTE can control priorities of sms, data, voice, etc much better than GSM/CDMA. But it will be awhile before LTE replaces (if ever) 3g completely...HSPA+ will keep the US on 3g for a few more years at least.