The same kinds of people who admonish you for taking an elevator because it's bad for the environment, even when your legs are sore from cycling to work.
I guess it's a kind of armchair environmentalism: it's easy to "go green" by switching to canvas bags (which _does_ help, a little), but then not make large, more sacrificial changes that would _actually_ benefit the environment. I don't know. It's tough, though, because to a large extent we're forced into lifestyles that are bad for the environment. You can't just decide to stop using the electricity produced by the coal power plant in your city, for example.
This -- a lot of the environmentalist movement is more about signaling that you care about the environment (turn the lights off, use one sheet of toilet paper) than actually investing the time or energy into figuring out the most cost-effective changes to make (stop flying as much).
This is true about almost all human groups/movements though, and I'm not sure that environmentalists are more guilty than average.
I have a smaller carbon footprint than most, I guess, though usually for selfish reasons. I ride a bicycle because I don't want to spend money on a car - either purchasing or maintaining one. For me it's a luxury item that can wait. Plus it's a time saver to do some cardio and get to work at the same time. Similarly I'm interested in these hippy "sustainable" house building techniques mainly because they're cheap and I want to avoid a mortgage.
But people who drive to work everyday still try and lecture me over nothings like putting a laptop on standby overnight.
Similarly, I drive an old SUV to work that gets terrible gas mileage because the environmental impact of prematurely switching to a new car is so much worse when I take my commute length into account.
I live within 8 miles of the office and subsequently put about 4,000 miles on my vehicle annually. I consume less fuel than 90% of the people in the office with new 'environmentally friendly' cars who live 40 miles away, yet it is I who receives the rash of shit. /rant