That argument won't fly. The First Ammendment protects you against the government. It does not give you the right to speak your mind while ensconced on private property you don't own. Note that the charge was tresspassing--which means a) entering onto private property uninvited, or b) remaining on private property after being asked to leave.
Imagine a mall owned by Christians, who might wish to expell a group of prosletyzing Hare Krishna. The right to property would be the issue, no freedom of speech or freedom of religion--and the latter would have to apply to all parties equally in any case.
You are incorrect. Multiple courts have maintained that while the property is private, its use is public and they cannot discriminate.
I can't speak to the legalities, but my sympathies are with the mall.
I'm old enough to remember trying to push past dozens of weird religous groups to get to my gate in the airport. A mall is a convenient, privately-owned place to shop - they should have the right to throw out anyone they want, for darn near any reason!