Why doesn't the Imam bear the burden of the first Tashahhud for the worshiper if the worshiper intentionally omits it, or if the Imam stands up quickly for the third rak'ah and the worshiper follows him, by analogy with the Imam bearing the burden of the glorifications if the worshiper omits them?

The tasbih (glorification) during bowing (ruku') and prostration (sujud) is considered mandub (recommended) by the Malikis, not sunnah (a confirmed practice). The difference between them is that there is no sin in deliberately omitting a mandub act, nor does it necessitate a prostration of forgetfulness (sujud al-sahw). As for a sunnah act, one who deliberately omits it incurs sin and must seek forgiveness, though it does not require a prostration of forgetfulness. The Imam does not bear responsibility for it on behalf of the congregant except in the case of forgetfulness. If the Imam rises quickly for the third rak'ah, the congregant should leave the rest of the tashahhud and follow him, as he (the Imam) is excused.