I am all for the free flow and representation of ideas and free speach of those ideas. But I am not for the forced availiability of mediums. That said, I don't support evolution, but supression of ideas is stupid but not illegal. Matter of fact, suppression of any form of conflict is ridiculous and dangerous. Additionally, you would think that Christians would support the free flow of ideas seeing what protestants went through around the time of Martin Luther and the 95 thesis. Unfortunatly, that is not the case.
Additionally, from a business standpoint, the IMAX theatres might be trying to attract the family entertainment market. Basically as I see it, they might attract more customers with family entertainment than with movies with an evolutionary spin to them.
I should also mention that many companies get blackmailed like this all the time as well. Particularly by fringe groups like Rainbow/PUSH/REV. Jesse Jackson. And sometimes it is easier to pay them off or just do as they request. Horrible as this sounds it is the truth.