The two are fundamentally different platforms. Twitter is an open space provided as a free service to the public at large, while a lecture is a fundamentally closed platform (only the lecturer speaks) only open to those enrolled in a specific class.

That said, even in the example you give it is not always considered appropriate to remove people causing disruption. Consider, for example, a conservative speaker giving a guest lecture being protested against -- often loud enough the the lecturer needs to shout to be heard by his audience, even though protesters are outside.

Here is a relevant video of Ben Shapiro giving a lecture at Penn State: https://www.youtube.com/watch?v=JLpa6fsTxCA