Why cannot put the small towel into the bathing pool of Japanese hot springs (onsen)?

For foreign visitors, the first question when they come to Japan probably will be why there are so many Japanese public bathings.
Let's start with most popular top three public hot springs of Tokyo. First of all, is “LaQua Spa” near “Tokyo Dome” where is able to improve and maintain good health, also high effective in skin care. Apart from featuring real hot spring onsen, many beauty services have been provided, therefore “LaQua” is very popular for woman visitors. The second hot spring will be “Toshimaen Niwanoyu” where is able to have relaxation space while having beautiful view of Japanese style garden. The third one will be “Ooedo Onsen Monogatari” where has Indoor bath, open-air bath, and sand bath. These three onsen bathings are filled with natural hot-spring water with large facility provided, therefore they are suitable for everyone with different genders and all age groups.
【Spa LaQua】
■Address:1-1-1 Kasuga, Bunkyo-ku, Tokyo
■Hours:Open from 11:00 AM to 9:00 AM the next morning (open 22 hours/day)
■Closed:Open year-round
When calling from outside Japan, replace the first 0 in the telephone number with 81.
豊島園 庭の湯(Toshimaen Niwanoyu)】
■Address:3-25-1 Koyama, Nerima-ku, Tokyo
■Hours: Open from 10:00 AM to 11:00 PM(cut-off time:10:00 PM)
■Closed:Open year-round
When calling from outside Japan, replace the first 0 in the telephone number with 81.
■Address:2-6-3 Aomi, Koto-ku, Tokyo
■Hours:Open from 11:00 AM to 9:00 AM the next morning(cutoff time for admittance: 7:00 AM)
■Closed:None scheduled
When calling from outside Japan, replace the first 0 in the telephone number with 81.
Indeed, these hot springs are welcome for every one and international visitors, however there are some etiquettes and rules you might need to know before use the hot springs. The very basic instructions have been listed as below:
1.1. Please use the hot spring water to clean the body in washing area before enter the onsen pool by using the washbowl. (the reason is too make sure the onsen water is clean for everyone also let your own body to get used to the water temperture).
2.2. Please keep the small towel out of the hot spring water.
3.3. Please tie up your hair to prevent the hair touch the hot spring water.
4.4. Please do not wash or clean your personal underwear or socks in the hot spring area.
5.5. Please do not leave the bathing area while the body still in wet, please use towel gently off excess water before entering the changing room.
Knowing the lists will provide you an enjoyable hot spring experience.
These rules and etiquettes might casued unbelieveable perception for the foreign visitors, but this also reflects on Japanese sanitary habit. From the past, basically everyone used public bathing due to there wasn't bathing facility in every house of Japan history. On the other hand, public bathing was becoming “public asset”. There were some people washed and cleaned their underwearing in the puclic bathing in the past. However, in present day everyone shares the bathwater, so in order to make sure everyone has clean water and comfortable environment, different kind of rules have been established.
The etiquettes and rules of public bathing has reflects Japanese speical philosophy “everybody enjoy the hot spring bathing for import physical and mental relaxtion without making troubles for others”. Hence, please soaking yourself into the warming hot spring, and experiencing and enjoying the time of special Japanese bathing culture. We would describe this as “wash off the dirt, socking your body into red like a shrimp, afterwards your life will be brand new and ready to begin.”