I think that's how it's implemented rather than a limitation.
I call it a limitation simply because reading vertical text is very unnatural and difficult. If anything, Windows and Qt should have had an option to still display horizontal text no matter the tab location. GTK2, fpGUI and MSEgui can do that.
I guess to fix this, Lazarus will then have to create a custom PageControl type widget for use in the IDE - after all, they already have a few custom widgets specifically designed for the IDE.
I remember a year or two ago I installed a Lazarus IDE add-on that showed the open files in a listbox type window. It allowed dragging files up or down to reorder them, did some filtering etc. Maybe that could be a suitable work-around for the LCL-QT and Windows users. Unfortunately I can't remember the name of that add-on, but I'm sure with some Google searching or a post in the mailing list you should be able to find it.