Its a feature. On the wiki it says:
Trees can be climbed in both fortress and adventurer mode. In densely wooded areas, the overlapping tree crowns can form a continuous canopy that can be traversed by walking, climbing and/or jumping. Both types of branches provide floor-like support for walking, but twigs are too frail to support the weight of a dwarf. All kinds of trunk tiles are treated as solid barriers, except trunk tips - they can be walked and jumped over.
I couldn't find any source where this is concerned as an issue so its not really a problem.
In adventurer mode you can climb them yourself by pressing H to first hold on it and then moving by pressing the movement keys, but I guess you already figured out.
"tree" on dwarf fortress wiki