You don't need to rent a whole sim to live in SL. You can pay under $75 a year & get a free cottage-size home with your premium account. Then if you want to build big stuff, you can use a free sandbox.
If you want to give that money to a homeless shelter instead of using it to play in SL, you can do that. But most people like to spend some money on recreation & leisure activities for themselves. Maybe instead of going out to movies & drinking Starbucks coffee, they spend $30-$60. month on Second Life. That's not too much money for most people who own a personal computer.
Many of us struggle to make ends meet financially, some more than others. But that doesn't mean people who can afford to spend money on having a little fun should feel guilty about doing do. Some sims in SL also provide groups of people opportunities to interact with each other & even learn stuff, so owning a sim or region may not be a solely selfish decision.