save only $5 for suspending a line!

Actually your paying $10 a month to keep the line and number till the person returns.  I agree it's high, but the alternate is cancel and port the number to google voice to save it.  The phone would of course have to be paid off as Next installments would be due in full.  And $45 to reactivate the line later.  

Wow! So it is $15x3=$45 to just leave things as it is, and $10x3+$45 = $75 to not have any service for 3 months just to keep the number? In which universe does this make sense?

Technically, the line costs $40, but because there is no "contract" on the line you are getting a $25 discount. So, you're actually only paying 25% of the line cost by getting that additional $5 off. Also, you aren't charged for turning the line back on with a reduced cost suspend. The $45 activation fee only applies if you cancel the line by porting to a different, possibly cheaper service, then port back in later. 

So if it were an unlimited data plan line, where the cost is $20 per line - the reduced cost suspension would be 25% of $20 = $5/month?

@Melindablue is my calculation correct in this regard?

I was talking specifically about the plan previously mentioned, where you're actually paying $40 w/ a $25 discount. The cost for a reduced cost suspend is $10 no matter which plan you're on, I was just explaining how that plan worked.

And specifying that you would not be charged afterwards to turn the line back on. 

So how do you justify paying 50% of the cost then on that regard? I honestly find this preposterous, at&t charging whatever is fancied apparently for the convenience to keep the number, especially when the user is not using any of the bandwidth. Is this cost justified by the service provided?

You're paying to keep your phone number, which is highly convenient. Upon researching, I found that Verizon and TMobile charge the same, Sprint only charges $1.01 less.