Somewhat true, but it depends on the country and the class of train. Rick shows a table of reservation fees, here.
In some countries, Italy, for instance, the fastest trains (EuroStar) are considered premium trains and require a surcharge to use them with a pass. Tickets for those trains come with reservations, which you get with the passholder surcharge ($15?). They call those "reservations". However, the seat reservations with an IC (InterCity) are not required and are nominal priced. Slower, local trains aren't reservable. You can ride all of those you want with just the railpass.
In Germany, seat reservations are only required on a handful of fast trains (e.g., ICE Sprinters) and are a nominal €11,50 per seat (2nd cl. €16,50, 1st cl.). On other express trains, seat reservations are sometimes advised, but not required, and are €4,50 (2nd cl, €5,50 1st cl.). Again, local trains never require reservations.
For Thalys (Paris, Brussels, Köln, Amsterdam), which are considered premium trains, the surcharge is quite high and comes with a seat reservation. Seat reservations for ICs in those countries are nominally priced.
Reservations are less expensive in Europe or on the Rail company's websites than from a reseller like RailEurope.