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Why do you call then if you almost certain that you are beaten? Is that still +EV?

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In addition to what others have already pointed out, if you fold on the end to a scary river card too frequently you'll discover you get bet at a lot more frequently when scary cards hit. Even bad players frequently notice someone who will fold on the end too much. Now when they get the urge to bluff they'll be more likely to do so against you. And as HEPFAP points out, worst of all, some players who are really passive and never seem to bluff now may decide to try a bluff, but only against you.

That's not to say you should always call - sometimes you can be virtually certain you're beaten and you can save a bet. Also, note that putting in one more bet to call is much more applicable to a heads up scenario on the end than it is multi-way. If one player bets, he could be bluffing, but when another player calls him, it's pretty likely at least one of them has something.

edit: Also note that if you can't decide how often to call you're probably doing okay if you just call in the big pots and fold in the small ones. What counts as big? Eh, I don't know. But some combination of your gut instinct and whether the pot is a certain size should guide your decision. Developing a feel for these marginal situations is probably what separates the break-even / small winners from the solid winners.