If you know people using a septic system, you may have noticed that they are extremely careful about what gets flushed down the toilet. In many cases, they just want to be sure nothing inappropriate gets flushed, but others may actually tell you to avoid putting toilet paper down the toilet, for example, even if it's septic-safe. Given that septic tanks are designed to handle things like septic-safe toilet paper, it may seem strange that these people are trying to prevent those very things from entering the septic system. However, this could be for a number of reasons.
Maintenance, Repairs, and Pumping Costs
Their budget may require them to stretch out the time between maintenance and service appointments for as long as possible. Like any home plumbing fixture, services like maintenance and repairs do cost money — this shouldn't be a surprise — and the owners' budget might be tight. Septic systems use tanks to hold the waste, and if you put too much down there, the tanks can fill up quickly. The pipes leading to the tank can also clog if you stuff too much toilet paper down in one flush. So, it's understandable that the owners would want to reserve money for maintenance and not have to worry about calling for repairs or for pumping that's needed earlier than normal.
Replacement of Older Tanks
If they're using a very old septic system, the time to replace the tank might be approaching, and they want to save their money for the new tank installation instead of constantly spending on repairs because someone else decided to flush too much down. A nice new septic tank and pipe will handle regular use well, but first, the owners need to be able to buy that tank and pipe. So, they're going to be extra careful about what enters the tank as they save up.
They and You May Have a Different Idea of What's "Too Much"
It could be that the septic tank is doing well and that the owners call for maintenance service and repairs whenever they're needed — but the owners have learned that guests who need to use the bathroom have a vastly different idea of what can be flushed down. If you're at someone's home and they give you very strict rules about flushing, it could be that they've had so many guests flush things that clogged the pipes and damaged the tank that they're no longer taking chances.
Whether the owners are saving up for a new tank or they just don't want to deal with others' concept of what can be flushed, it's helpful to listen to what they're telling you. It's also a helpful lesson if you ever move into a home that has a septic tank as you'll know to be careful about what you flush lest you need to call for septic system service a lot earlier than you thought you would.