I’ve never seen such a sign, but I’ll take a guess what it might be referring to. Here in California, the definition of a freeway does not have anything to do with number of lanes, speed limits, the presence of freight traffic, or any affiliate with the National Highway System. Instead, it is defined in the California Vehicle Code section 332 as:

a highway in respect to which the owners of abutting lands have no right or easement of access to or from their abutting lands or in respect to which such owners have only limited or restricted right or easement of access.

This roughly corresponds to what the Wikipedia describes in its page on "controlled access highways", a term which includes the California and USA federal government’s term of freeway or the eastern US states’ term of expressway or the British motorway. That is, a road which all ways onto and off of the road are carefully crafted.

There are many roads in California and the United States which will meet the requirements outlined by the Interstate Highway standards, and will look and feel like an interstate freeway, from the signs and lane markings and shoulder sizing.

But none of that matters for the California legal definition of freeway. Indeed, some freeway-looking roads will have signs that say “end freeway” or “start of freeway” with no other visual cues. And this is because the California Department of Transportation (CalTrans) has not acquired the property rights from adjacent landowners to prohibit building driveways onto the public right-of-way.

To clarify, a right-of-way is not an individual right like free speech or freedom against unreasonable searches. Rather, it’s a legal term referring to a property right, namely a grant of access on/over some piece of property to go across it. In the case of public roads, the property right is held by a public entity, and that means the public can use it.

A feature of public rights-of-way is that any adjacent private properties can connect to and travel upon the right-of-way. The rationale – to oversimplifying things – is that if the public entity could deny the right to build a driveway, then a lot could end up with zero driveways, making it impossible to enter or exit, which makes the property near worthless. It is an age-old rule from English Law that rendering property worthless is bad, so private property rights necessarily comes with an implicit ability to connect to adjacent public rights-of-way.

But property rights are a bundle which can be sold separately by their owner, akin to how many suburban property owners don’t own the rights to minerals underneath, with the earlier developers selling that right to someone else. And so the state – through CalTrans – or the city or county can buy – sometimes through eminent domain – the right from the property owners. Thus, the properties along a road might – unnoticeable it the naked eye – not be legally allowed to build a driveway, having shed that legal right away by sale.

To that end, it’s possible that a sign warning against illegal driveways is the state’s way of preventing future land owners from trying to build such driveways, since those owners would lose in court. If the state has acquired such rights, it’s usually because the road is planned to become a freeway or expressway (a limited-access road, in California terminology). So far as I’m aware, in California the right is only ever acquired for state roads, with the sole exception of the expressways in Santa Clara County, because they planned well ahead in the 60s.

Other states may be similar, by extrapolation.

TL;DR: OP’s state might be hedging their bets to build a freeway, and wants to prevent future legal issues with landowners, since the state knows it would win those cases