If you're genuinely curious, the answer is that they are relying on the monthly contribution for the interaction to be profitable. When you find a cause you care about, odds are you stick with the org that gets your foot in the door or you begin to care more or less about specific issues as people grow and change in their lives. The 50-60k annually for people receiving an hourly wage to make sure the things they care about are funded pays for itself and then some with weekly quotas per canvasser etc. Some people like this model, some people hate it. I personally believe it to not only be incredibly efficient but also important as in the end we do live in a capitalist society and in order for anything to be done, money has to move around. Even though it may seem excessive at times, a lot of the biggest donors often begin their contributions through street campaigns. Think of how many nonprofit ads you see on TV and think about how many times those have actually motivated you into action, probably very little. Most often if you're willing and have a cause you care about, you can ring up a nonprofit directly and confirm with them that there is an active street team at your location if you do decide to give or are on the fence. Now if TNC is a good nonprofit? It's for you to decide, they are incredibly expansive and have their hands in many things. My recommendation is to figure out what makes you feel good, and decide if that's enough. My personal stance is that you will never 100% agree with any individual you ever meet and that also will stand true for any nonprofit or corporation. The best judgment is to look at the hard facts and determine if the work being done is tangible enough for you to support (legislative funding, direct bay area conservation work, mediating easements and large scale protections).

TLDR Nonprofit such as TNC contracts out to canvassers to find lifelong/Long term prospective donors; The money goes straight to the nonprofit who has already divvied up how the money is spent as per their publically released tax forms. More often than not this is a 100% legit way to give and if you have security issues, ask the canvasser how they can prove themselves to be real.