To properly answer your question, we go back to the SPEN plan from the late 70s, to when it was published in 1980, the specs for SPEN 1 intended for it to be in CTCSS mode. When going to SPEN 2 (at the time, a newly created national law enforcement emergency channel), the CTCSS should be completely eliminated and the base and mobile should function in carrier squelch.
Likewise, SPEN 3 was a CTCSS channel, and SPEN 4 was intended to be a multidisciplinary frequency with no CTCSS.
The ability to program radios with either a PROM or software emerged in the late 80s and was mainstream in the early 90s. There were many people now selling and working on those radios, and most of them had never even seen the SPEN plan (or the complementary JEMS plan for EMS). They began to program the channels intended for carrier squelch with CTCSS, and the CTCSS channels with tone transmit, but carrier squelch receive. NONE OF THAT WAS EVER INTENDED. Here was the problem when cheaper products like Radius mobiles hit the streets: technicians at the time (like me) were getting complaints of multiple departments not being able to communicate with each other on the SPEN channels. What was happening was one agency went to Big Guy Two Way and the other agency went to Tommy's Two Way. One had been doing SPEN radios all along, the other was not a bad shop, but they were new and didn't know the specifics of how everything should have been programmed.
The solution was to program everything with 131.8 Hz in transmit and, when in doubt about the region, carrier squelch in receive. After that, everyone was able to hear each other, regardless of what their respective shops did.
That's how it started. But:
SPEN 1 = Intended to be full-time CTCSS
SPEN 2 = Intended Carrier Squelch
SPEN 3 = Intended to be full-time CTCSS
SPEN 4 = Intended Carrier Squelch
In fact, the caches of SPEN portable radios the state had in 1986 were red MT500 portables. They were internally strapped with diodes to turn the tones off on channels 2 and 4 and open the receiver to carrier squelch.
All that said, this was all thought up 40 years ago. I would expect some changes have been made over the years.