Syllabification in English is an uncertain and contentious topic. I'm sure you could find people who would say they syllabify "disable" as /dɪˈseɪ.bəl/ instead. In fact, the OED shows this as a possible syllabification of the /s/ (though oddly enough, only for the American pronunciation: it says " Brit. /dɪsˈeɪbl/, U.S. /dəˈseɪb(ə)l/).
It's easier to be more certain about the syllabification of "disposable" for two reasons.
First, because the /s/ precedes a voiceless plosive /p/. In English, voiceless plosive consonants exhibit what is called "conditioned allophony", which means they are pronounced slighly differently in different situations. In particular, a voiceless plosive at the start of a stressed syllable will be aspirated"—it has an extra puff of air—while a voiceless plosive after an /s/ in the same syllable is never aspirated. This is a phonetic or phonological clue to the syllabification. I have never heard a speaker use an aspirated "p" in "disposable", only unaspirated "p". This indicates that the /p/ is not at the start of a stressed syllable, which rules out the syllabification /dɪsˈpoʊ.zə.bəl/. And I think it's obvious that /dɪspˈoʊ.zə.bəl/ seems wrong (there are more technical theoretical arguments against this syllabification, but I don't know how to present them).
Second, because "disable" is obviously composed of two parts, while the composition of "disposable" is less blatant. Etymologically, it has the prefix "dis-", but it doesn't relate in any obvious way to the words "pose/posable", so it's likely that many speakers don't make a conscious connection between the words.
The s in "disable" is more tricky to syllabify because there aren't any major types of allophony like plosive aspiration that would tell us which syllable it belongs to, and the compositionality of the word is, at least in my opinion, more clear, so some people might think of it as "dis-able" and pronounce it accordingly in two separate syllables corresponding to the constituent units of meaning.