Actually, as a former inspector, I can tell you that some roofs can be easily damaged if the sub structure is deteriorated or water damaged and an inspector walks on the roof - such as tile or slate. Also, 2nd story roofs are often steep and dangerous. From the binocular standpoint, using numerous vantage points, it's very possible to see flashing issues, vent problems, plumbing stack insertion issues, torn shingles, etc. When coupled with an observation of the attic space, it's very likely to ascertain water penetration issues and roof problems.
On a side note, I know each state is different, but in Texas, the State Standards of Practice for inspectors address this issue:
"(f) Specific limitations for roof covering. The inspector is not required to:
Â .....(2) inspect the roof from the roof level if, in the inspector's reasonable judgment, the inspector cannot safely reach or stay on the roof or significant damage to the roof covering materials may result from walking on the roof;"