Is this a modern phenomena, or were greek gods considered to be almost interchangeable with roman gods during the time of the romans as well?
This is a really, really, really complicated topic I am currently writing about. The key idea is how the Romans viewed their gods. I describe it like this: Imagine each god is a rich guy, who you really want to talk to for some reason. You know their addresses, but they're rich, so they have a lot of different houses. Imagine each of these houses is an aspect of the god. By going to each of their houses, you have a better chance of contacting that person, either by finding whoever you were looking for, or by finding someone who knows them. These "houses" could be physical locations, or certain rituals, and Romans never really knew which house worked.
Basically, each god had multiple aspects that were taken, or "romanized" from other cultures. In particular, my study concerns the Romanization of Carthagian gods, and the best example I have is Juno/Hera/Tanit/Astarte/Uni. The names may get a little complicated here, so bear with me.
Juno is the motherly god of the Roman pantheon, and what we commonly consider the Roman version of Hera, and married to Jupiter, or Zeus.
Hera is the motherly god of the Greek pantheon, and married to Zeus.
Tanit is the head female goddess of the Carthagian pantheon, married to Baal Hammon, who is romanized as Saturn, or Kronos.
Astarte is the most important female Phoenician goddess, who lost popularity in Carthage to Tanit.
Uni is the motherly god of the Etruscans, who occupied Italy before the Romans.
The key here is that Juno takes aspects from each of the earlier deities. Literally, Juno took several forms and personality traits depending on how an area/person viewed her. For example, Juno Caelestis takes a very warlike aspect from Tanit.
In some instances, these aspects were literally stolen from their home cultures. In the case of Juno Regina, the Romans performed an evocatio*, where they invite the god(dess) to leave their old culture and become Roman. "Juno Regina, you who now inhabits Veii, I pray that you will follow us as victors to our city, soon to be your city, where a shrine worthy of your majesty will receive you (1).
To sum it up, it isn't that the Greek and Roman gods became interchangeable with each other. The Romans saw the Greek Hera, and decided that she was similar enough to Juno to consider her an aspect. Because many of the writings we have use the names of gods interchangeably, and the Romans and Greeks together make up so much of Western civilization, over time the two pantheons have been considered interchangeable, even though they aren't. Imagine it this way: Greek gods are a cream sauce. Roman gods are clam chowder. Clam chowder takes cream sauce, and adds a bunch of stuff to it to make it something new. Cream sauce can stand on its own. But if you only take a quick glance, all you see is a big pot of white stuff.
This isn't a very well written answer, because it's late and I'm tired, but I'll try to edit it and make it better tomorrow afternoon. Feel free to ask follow up questions.
Sources:
This is a quote form Livy 5.21, found in Foreign Cults in Rome by Eric Orlin
La Romanisation des Dieux by Alain Cadotte
Religion in the Roman Empire by James Rives
In the case of the Greek Gods it is because they were in Italy before Rome and so were already familiar at the outset of the empire. There were Greek colonies in Italy, Naples, Syracuse, Bari etc. Even the Romans referred to southern coastal areas of Italy as Magna Graecia.