Because again, the orcs didn't know the land belonged to the night elves, or that the night elves even existed. They did not know they were invading anything. They did not know the trees held any meaning to anyone as something other than a source of lumber. All other races besides night elves cut tree down for wood to build homes. Is it bad to cut down ANY tree regardless of where it is? That would make every race other than night elves "monsters". Since the orcs had no way to know the land was sacred how are they evil for gathering wood to make their homes?
And later during the night elf mission humans and orc work together and cut down trees to build up their bases in preparation for a conflict with the demons. Tyrande and her elves kill both. Later during the last mission all 3 factions work toegther again Archimonde, the orc and human cut down trees for lumber and the night elves don't say anything about it.