Spirit Airlines: Carry-on weight limit?

You are correct in observing that Spirit and many other airlines do not publish a specific weight limit for carry on bags. Probably because this is a limit there's hasn't been a notable reason to enforce. Some exceptions are described here.

The most specific practical limit you must comply with is any limit of the overhead bin. I don't know what they generally are, but I've seen the stickers and some posts indicate 70-90 lbs. The workaround for that is to put the bag under the seat in front of you.

10kg is not particularly heavy, but at the upper end of the above limits. Note, while it will likely go unnoticed, if the crew does notice and thinks it might be to heavy to be safe, they will require you check it. Once that decision is made, there is no point in arguing. You only options are check the bag or depart the aircraft.

Indeed, this is common now among US carriers. Since you have a size-limit, though, there will be practical limit eventually. Given all the restrictions and additional rules for check-luggage, this is good for people to have flexibility.

10kg for a carry-on is very reasonable. Most airlines that do specify a weight limit usually put it at 22 lbs or 10kg. On very small planes I have seen lower limits down to 5kg when flying around Asia but that is fairly exceptional even and it causes problems because most of these airlines also have a rule about not putting valuable and electronics in checked luggage, so if you carry a couple of cameras, a few lenses and a laptop with power-brick, you are easily over 5kg or even 7kg.