You seem surprised, but this is an indicator that you're not reading a good article. They are covering something and don't even accurately describe the proposal by NHTSA. But thankfully they link to the proposal itself. So here's the summary:

> This document proposes to
establish a new Federal Motor Vehicle
Safety Standard (FMVSS), No. 150, to
mandate vehicle-to-vehicle (V2V)
communications for new light vehicles
and to standardize the message and
format of V2V transmissions. This will
create an information environment in
which vehicle and device manufacturers
can create and implement applications
to improve safety, mobility, and the
environment. Without a mandate to
require and standardize V2V
communications, the agency believes
that manufacturers will not be able to
move forward in an efficient way and
that a critical mass of equipped vehicles
would take many years to develop, if
ever. Implementation of the new
standard will enable vehicle
manufacturers to develop safety
applications that employ V2V
communications as an input, two of
which are estimated to prevent
hundreds of thousands of crashes and
prevent over one thousand fatalities
annually.

As to your questions,

> what about the safety benefits of [the] proposed technology? ... How's this supposed to increase safety?

From the executive summary, the benefit of this is that it provides more information without requiring line of sight to other vehicles. It can "see" through cars allowing it to perceive dangerous situations around corners and behind other cars. It provides more information like breaking force and path prediction so that more corrective action can be taken. If vehicles are updating each other with steering inputs and break inputs along with path prediction its hard to not envision how this could increase safety. A car can react to emergency breaking far far faster than a human can.

From the linked proposal, there's a whole section VII Estimated costs and Benefits. Section D there is Estimated Benefits with subsections 1. Assumptions and Overview 2. Injury and Property Damage Benefits 3. Monetized Benefits 4. Non-Quantified Benefits. From the monitized annual benefits section,

> Table VII–38 provides the
undiscounted annual fatal equivalents,
monetized benefits, and property
damage and congestion savings of the
proposed rule from the year 2021 to
2060. As shown, by Year 5 the proposed
rule is estimated to save 129 to 169 fatal
equivalents totaling approximately $1.3
to $1.6 billion annually. Approximately
12 percent of the monetized savings,
$176 to $237 million, are from the
estimated reduction of property damage
and congestion. By the year 2060, with
V2V fully deployed, the proposed rule
is estimated to save approximately 5,631
to 7,613 fatal equivalents annually.
Finally, the total associated monetized
annual savings would range from $54.7
to $73.9 billion. Of these savings, $7.7
to $10.6 billion is estimated to be
property damage and congestion
savings.

> Also, who is going to pay for these boxes

Section VII (Estimated Costs and Benefits) B (Quantified Costs) 1. (Component Costs) has your answers! Physical hardware is estimated to add $245.79 for one radio and $347.18 for two radios with an estimated $17.80 installation fee. The added weight is expected to add $0.19 - $6.97 in fuel costs per year. Summarizing "The cost per new vehicle would range from $135 to $301...". As to the who, presumably the consumer.

> What is the penalty for not installing one?

From Section I (Executive Summary) the wording is that "to require all new light vehicles to be capable of Vehicle-to-Vehicle ("V2V") communications, such that they will send and receive Basic Safety Messages to and from other vehicles. This seems to be a regulation aimed at manufacturers, and non-compliance would probably come from marking the fleet of manufactured cars as not road legal.