Well, unless you are absolutely sure that the branches you graft to other people's trees are disease free, you are definitely part of the problem, not the solution.
The other problem with guerrilla alterations is that some people find it upsetting when someone grafts a Golden Delicious onto a native Malus - even in Seattle. The Pacific crab apple, which provided the "apples covered with grease" that were stored and eaten in the winter by the western tribal peoples are small, edible, and unimproved by the addition of branches from domestic cultivars.
I planted my entire front yard with indigenous species, most of which are edible, at least in some portion. I enjoy shadberry, Oregon grape pie, and native wild grape, as well as the delicious "limonada" made from other edible native fruits. I suggest that as a better route. How many people spend far too much time irrigating, fertilizing and introducing demanding plants when there are literally dozens, if not hundreds, of native plants in almost every region? Manzanita or salal, anyone? Native bramble berries?
I had to remove a number of my native plants because people trashed them trying to steal the edible portions and didn't care if the plant survived or not because they lacked morals. I hope they weren't trying to be permaculture guerrillas because they seemed to be destructive vandals and nothing more. I replaced the destroyed edible native plants with inedible or less known edibles that are known to attract pollinators and have different seasons. I now have more than a half dozen different native bee species and I see almost every hummingbird that passes through the area.
Tragically, in the area where I live a great deal of damage has been done by people entering reserve land and planting wildflowers from other continents and regions, and by stripping and eating the native edibles. Grow your own, folks, don't over harvest wild plants. On the other hand, as Jason noted, many people seem unaware that many urban park ornamentals are also edible. That resource is terribly underused in this area, although I have encouraged the homeless to eat the madrone berries late in the season, after the birds have dispersed enough seeds, and to enjoy the domestic plums which are, as Jason noted, extremely edible - in fact, downright delicious.
