because that is acquired, but originally they were photosynthetis. That’s like if you asked, why to say, that dolphins are mammals, when they live in water? Why not say, they are fishes?

As far as I know, photosynthesis is not the main criteria for an organism to be classified a plant despite it being perhaps the most prominent feature of plants. Plants are classified by a combination of several biochemical features, such as typical metabolic pathways, genes for clorophyl(?) and the composition of the cell wall etc. Also, I think even such parasitic plants that have no visible clorophyl still have the genes needed for its production (and most can under some conditions produce little of it).

I’m not 100% certain of this, though. Let’s see if someone else knows more.

Interesting question :o))

http://apps.isiknowledge.com/full_record.do?product=WOS&search_mode=Refine&qid=3&SID=U2P4eCMFn@75LKc1Olo&page=1&doc=1
(Vidal-Russell R and Nickrent DL (2008) The first mistletoes: Origins of aerial parasitism in Santalales)
They apparently do have chloroplasts
DNA sequences from nuclear SSU and LSU rDNA, and chloroplast rbcL, matK and trnL-F from 39 santalalean taxa were obtained.

But anyway, the genes for Chl biosynthesis do not have to be coded by the chloroplast DNA, but never mind :o))