Pete, by that do you mean they cannot for some reason launch a vehicle which is capable of knocking out other orbiting vehicles?
It can, but won't be doing it well enough to be useful. The main reason is the enormous energy cost of any change in the orbital inclination in lower orbits, which is about the same as sending something into a high orbit. So you basically cannot pre-position the orbiter, you must launch it into the target's orbit. Of course you can aim for a collision from the outside of the oribital plane but as the shutfown of USA 193 demonstrates, launching an orbiter to do it is immensely stupid. Firstly, it's way more expensive than a missile warhead, and secondly its performance is very much inadequate due to weight penalty of wings etc. We had a spare SM-3 missile to shoot the 193, but we don't have a spare Atlas-V to loft X-37 or its follow-up.
Now what an orbiter could do is snatch something instead of knocking it (if you plan it 2 years ahead due to launch availability). Sadly X-37 a) is too small to snatch anything bigger than an ORBCOMM sat, b) cannot reach even GPS orbit let alone GSO, c) even if it could, would not be able to return without an upgrade to its heat shield. A more advanced spaceplane is enough of a possibility that my college friends are pondering allocating a kilo of mass for a barometric sensor and a plastite charge for sensitive birds.
What it comes down to, X-37 is a program without a sensible objective. This is why people on the Internet are trying to find explanations. I thought about it too for a bit. For example, it may be the frustration with the synthetic aperture on micro-satellites and the whole ORS thing (Operationally Responsible Space). Its proponents promised that they'll stockpile these little things, launch them on demand of the warfighter. It turned out to be a bad idea, little sats can't do the job of big ones. Maybe someone thought, very well, we'll launch the big one, then we'll return it... launch again to a different orbit (remember the orbital inclination thing I mentioned). This way it can be big and expensive yet responsive (provided we stockpile launchers too). It is possible... But you know how between stupidity and malice you should bet on stupidity? The explanation that USAF just wanted a spaceplace seems simpler and more direct to me.
-- Pete
