Note that this is a (very slightly modified) quote from Eutropius' Breviarium historiae Romanae, 2, 12. Full sentence:
Postea Pyrrus, coniunctis sibi Samnitibus, Lucanis, Brittiis, Romam perrexit, omnia ferro ignique vastavit, Campaniam populatus est atque ad Praeneste venit, miliario ab urbe octavo decimo.
The addition miliario ab urbe octavo decimo is apparently not intended to say where Pyrrhus moved, but to remind the reader how close Praeneste was to Rome: “at the eighteenth milestone.” You could translate it as: “… he came to Praeneste, eighteen miles from the City.”
If the author had wanted to express somebody had approached the such-and-such milestone from Rome, he would have used the accusative as you expect. You can see this in 1, 5:
Romanos saepe vicit, usque ad quintum miliarium urbis accessit
He “advanced up to the City's fifth milestone,” i.e., he came as close as five miles from Rome.
However, it still seems unusual to use an ablative of place without a preposition. But this is not an accident: Eutropius (whose concern for milestones rivals that of any modern-day project manager) regularly uses miliario this way, e.g. 1, 4:
Albanos vicit, qui ab urbe Roma duodecimo miliario sunt, Veientes et Fidenates, quorum alii sexto miliario absunt ab urbe Roma, alii octavo decimo, bello superavit
Sexto miliario ab urbe abesse seems particularly weird, but note that you can write sex milibus passuum abesse (“to be removed by six miles”), or, of course, sex milia passuum abesse (“to be six miles away”).
We also find (1, 8):
Postea Ardeam oppugnans, in octavo decimo miliario ab urbe Roma positam civitatem, imperium perdidit.
But I think this is because of positam (“situated”), which clearly calls for an explicit expression of place.
In the other cases, I assume the author treats it like a fixed expression of distance. This does not appear to be a common classical expression (note that Eutropius lived in the 4th century), but it is clearly a particular favourite of Eutropius, although Google finds a few other hits as well.