1) It is a "safe" method.
2) It is an idempotent method.
A safe method (GET/HEAD) should not modify server state.
"In particular, the convention has been established that the GET and HEAD methods SHOULD NOT have the significance of taking an action other than retrieval. "
Whereas PUT & DELETE are also idempotent but not "safe".
> Naturally, it is not possible to ensure that the server does not generate side-effects as a result of performing a GET request; in fact, some dynamic resources consider that a feature. The important distinction here is that the user did not request the side-effects, so therefore cannot be held accountable for them.
I guess my point is that idempotence is the aspect that matters to clients - you can have side-effects if you really want, so long as the request can be safely repeated.
Of course, it's not actually-invisible: their next request might be a little bit faster.
"Same effect from 0 or more actions" implies "Same effect from 1 or more actions".