At first glance, the distinction between not doing craft and abstaining from craft seems semantic and trivial. Isn't not doing the same thing as abstaining? From a behavioral point of view, this is a good point. With Ralbag's emphasis on Toelet, meaning and benefit however, the distinction rings true.
To merely not do something is to practice arbitrary self denial and is to be condemned to failure. The overeater tries to not eat, the drinker tries to not drink, both rarely if ever succeed. To abstain is to have a superior purpose for the sake of which one chooses to avoid overindulgence in an enticing pleasurable activity. The successful dieter's non eating supports a health oriented lifestyle that justifies some loss of eating pleasure.
The principle applies with equal force to Issur Melacha. To attempt to simply not do melacha on Shabat is to engage in a futile battle of will, one that will succumb to the allure of weekend entertainment or 24/7 workstyles. To abstain from melacha is to refrain from melacha , precisely because one is aware of the superior benefit opened up by redirecting energy away from overindulgence in the material domain for the sake of involvement in superior domain of mind. Ralbag develops this idea in Toelet #10 and 11.
In Toelet 11 the Halachic aspect of Issur melacha alone is explored, therefore the Mitzva is formulated at the behavioral level in isolation-- to not do craft on the Shabbat. This behavioral aspect is the focus of the Lo Taaseh as it is presented in the pasuk “You shall not do any Craft". Lest the modern reader slip into the error of thinking that mitzvot can be understood at the behavioral level alone, Ralbag emphasises the need to review the toelet from the previous Toelet- #10.
התועלת הי״א היא במצוות. והוא מה שהזהירנו מעשות כל מלאכה ביום השבת. שנאמר ״לא תעשה כל מלאכה” וכבר בארנו התועלת המגיע מזאת המצוה במה שקדםThe 11th Toelet is in Mitzvot, this being that we are prohibited from doing all craft on the Shabbat day- as it says “You shall not do any Craft”. We have already explained the toelet which comes from this Mitzva in the previous section.
In Toelet 10 the end pursued by this non doing is explored via the aseh of Shevita-to abstain from craft on the seventh day. This abstention is rooted in the pasuk “six days shall you work and accomplish all your craft but the seventh day shall be a Shabbath dedicated to Hashem your God”.
התועלת העשירי הוא במצוות. והוא מה שצוה לשבות ממלאכה ביום השבת. שנאמר ״ששת ימים תעבד ועשית כל מלאכתך ויום השביעי שבתלה׳ אלהיך וגו,״. והנה התועלת בזאת המצוה הוא בדעות ובמדות.
The tenth benefit is in Mitzvot this being that we are commanded to abstain from melacha on the Shabbat day as it says "six days shall you work and accomplish all your craft but the seventh day shall be a Shabbath dedicated to Hashem your God". The benefit from this Mitzva is in theoretical ideas and practical affairs.
Clearly it is our insight into the toelet, benefit and meaning, of Shabbat in theoretical ideas and practical affairs that justifies abstaining from the allure of preoccupation with melacha alone.
But what is this insight that transforms Issur Melacha from a futile non doing into a productive abstention from overindulgence?