As a conference organiser, you are responsible for what happens on your stage. Your stage, your rules. This starts with the backdrop, the IT system and the lighting. You are also responsible if a speaker puts up unacceptable images, expresses extreme political views or delivers a poor presentation. I’m not clear why we understand the first two points clearly but vacillate on the latter. Your stage, your rules.

Is it reasonable for an organisation to expect a quality presentation from their stage? Completely. The quality, as well as the content of a presentation, is within the remit of those who have invited a speaker, whether the invitation is by reputation or dint of abstract submission. A presenter who has no structure, who simply lists facts without direction or bores the audience by their delivery is also under the remit of the organisation. Your stage, your rules. One can and should expect quality. Sadly, we only find out about the quality after delivery.
I’ve worked with various organisations including Don’t Forget the Bubbles to help speakers achieve a new level of presentation quality before delivery. The organisation committee decided that they wanted a particular quality and style of presentation from their presenters. They were clear in their invitations about this and offered all speakers a presentation coach to help them understand the issues and achieve this quality. The person-hours involved were significant, not least because very few presenters prepared well ahead of time but the subsequent quality of presentations was hugely improved. The joy of the presenters involved in this journey was also palpable. The OC and their audience were rewarded.
Some presenters believed they already were of (more than) an acceptable level of excellence. They did not feel the quality requirement or the offer of support applied to them. Their experience in repeated presentations has not been criticised and even noted that feedback has always been good. Please remember that “Thank you for excellent presentation, I very much enjoyed it,” is custom and politeness not educated feedback. Failing to engage with the organising committee’s request is interesting in itself, the gulf in quality and impact of those who did engage was obvious. Please check this post describing one such partnership. Is it reasonable to make such a request? Of course, the stage belongs to the Organising Committee. Your stage, your rules.
No organising committee accepts poor behaviour on the presentation stage. It is their responsibility to deliver excellence and if that includes direction and support for a presenter to achieve a higher quality of presentation, that is their right. Your stage, your rules.