Why is Tisha b'Av apparently stricter than other mitzvos--including Shabbos--with regards to rabbinical prohibitions for a choleh? On Shabbos, one who is in a state of choleh kol gufo (debilitating illness) or even tza'ar godol (severe pain) may do rabbinically-restricted melochos with a shinui for his comfort (source: Ribiat, The 39 Melochos, pp. 491-493). Also,
"A pregnant woman is permitted to take any medications, vitamins, or therapies to avoid any risk to her health or that of her baby. One may violate any melocho to save the life of a fetus, (even in the earliest stages of pregnancy)"
Even if one is ill, he must still observe the fast of Tisha B'Av to whatever extent possible.He may not eat more than what is necessary. [This is the basis of R. Schick's opinion.] Furthermore, he must still observe the other inuyim. As such, he is still considered someone who is fasting.
So basically I am asking, if even rabbinical matters surrounding Shabbos can be transgressed (with a shinui) for a choleh, then why can't the rabbinical enactment of fasting on tisha b'Av--to say nothing of the other rabbinical enactments for that day--be readily transgressed for a choleh? I understand one difference is the shinui, but is that all? If not, why did the rabbis allow more leniency for Shabbos (and, it would seem, most other mitzvos) than for Tisha b'Av?