Placing flagging tape along an existing trail is littering, pure and simple.
Respectfully, I would again argue context.
Tape for the purpose of marking a clearly well-trodden trail in summer is gratuitous I agree, but firstly, not all tape is there to mark the run of the trail, as mentioned previously; given the OPs example that elevation was marked (inaccurately or not) it may not have been mark the trail, but a property management boundary or a dozen other reasons unbeknownst to the passerby (it's not really a support for the position I'm defending, but I could understand how one would be choked if they had spent time flagging an area for a legitimate reason only to come back to find someone had removed it all, more so if they learned it was just because it conflicted with someone's outdoors aesthetic. Either party could conceivably call the other a vandal).
Additionally, trails that are easily followed in the summertime may not be so when snow-covered. An example that comes to mind is the Cheakamus Lake trail; every winter many skiers get lost skiing out of bounds on the south side of Whistler and, assuming they clear the substantial cliff bands and frozen waterfalls below the Cake Hole, end up along the Cheakamus River somewhere between the lake and civilization. This is enough of a problem that WB and the municipality have installed non-lingual signs intermittently in the bottom of major drainages pointing the way to the highway some 10-11 kms away. Since the trail is not heavily traveled in the winter due to its lack of car accessibility the bed is not obvious, especially when foliage is weighed down with snowfall. Flag tape that would seem excessive in the summertime might mean the difference between spending the night out or not for many. Admittedly, this may not be an issue for lower mainland trails without sufficient snowfall or isolation, hence my argument of the importance of context.
The overuse of flag tape (again, subjective judgement) could definitely be considered irresponsible, but the rash decision to remove tape when unaware of its purpose could also be construed as such, but have more serious consequences, ranging from merely frustrating to dangerous. Not saying that the worst is likely, only that there's more potential when expected trail markers are removed.
I don't personally feel that strongly about flag tape, just illustrating another position from the OP. I have no problem assuming it's there for a reason and ignoring it beyond postulating on it's purpose.