Why to repair 3G have to restart phone?

Try to switch airplane mode on and off may also help.
More such options you can try:
1) in the settings of SIM card to turn off and turn back a specific SIM card.
2) in the settings of mobile networks to launch a search for networks and manually connect to the correct back.

It was the cry of the soul, or really interesting technical details? )

Briefly, the reason is quite technical, and it sounds "savings".
Next should be a phrase like "reduce the cost of additional equipment, we achieve reduction in the cost of our services." As an engineer, I can't judge how much truth in it, but logic dictates that some correlation needs to be really.

In General, block the subscriber of the mobile Internet is possible in several spots, and the oak, but effective and cheap, requiring expansion of network nodes, that is as seen in: the money ran out - the Internet was gone prior to re-registration in the network.

And in another device with this SIM card, a similar situation?

You can learn (the answer to this question) to help the operator.
By the way: when I leave "in a minus" (a couple of times in 18 years happened), then the break was not observed (even in roaming, RF).
Here to call/(send a message) if negative balance (without additional options like "promised payment") is not obtained.
And while balance is greater than zero - easily.