Why is "extends T" allowed but not "implements T"?

There is no semantic difference in the generic constraint language between whether a class 'implements' or 'extends'. The constraint possibilities are 'extends' and 'super' - that is, is this class to operate with assignable to that other one (extends), or is this class assignable from that one (super).

Probably because for both sides (B and C) only the type is relevant, not the implementation. In your example

public class A<B extends C>{}

B can be an interface as well. "extends" is used to define sub-interfaces as well as sub-classes.

interface IntfSub extends IntfSuper {}
class ClzSub extends ClzSuper {}

I usually think of 'Sub extends Super' as 'Sub is like Super, but with additional capabilities', and 'Clz implements Intf' as 'Clz is a realization of Intf'. In your example, this would match: B is like C, but with additional capabilities. The capabilities are relevant here, not the realization.

It may be that the base type is a generic parameter, so the actual type may be an interface of a class. Consider:

class MyGen<T, U extends T> {

Also from client code perspective interfaces are almost indistinguishable from classes, whereas for subtype it is important.

We are used to

class ClassTypeA implements InterfaceTypeA {}
class ClassTypeB extends ClassTypeA {}

and any slight deviation from these rules greatly confuses us.

The syntax of a type bound is defined as

TypeBound:
    extends TypeVariable 
    extends ClassOrInterfaceType {AdditionalBound}

^{(JLS 12 > 4.4. Type Variables > TypeBound)}

If we were to change it, we would surely add the implements case

TypeBound:
    extends TypeVariable 
    extends ClassType {AdditionalBound}
    implements InterfaceType {AdditionalBound}

and end up with two identically processed clauses

ClassOrInterfaceType:
    ClassType 
    InterfaceType

^{(JLS 12 > 4.3. Reference Types and Values > ClassOrInterfaceType)}

except we would also need to take care of implements, which would complicate things further.

I believe it's the main reason why extends ClassOrInterfaceType is used instead of extends ClassType and implements InterfaceType - to keep things simple within the complicated concept. The problem is we don't have the right word to cover both extends and implements and we definitely don't want to introduce one.

<T is ClassTypeA>
<T is InterfaceTypeA>

Although extends brings some mess when it goes along with an interface, it's a broader term and it can be used to describe both cases. Try to tune your mind to the concept of extending a type (not extending a class, not implementing an interface). You restrict a type parameter by another type and it doesn't matter what that type actually is. It only matters that it's its upper bound and it's its supertype.

In fact, when using generic on interface, the keyword is also extends. Here is the code example:

There are 2 classes that implements the Greeting interface:

interface Greeting {
    void sayHello();
}

class Dog implements Greeting {
    @Override
    public void sayHello() {
        System.out.println("Greeting from Dog: Hello ");
    }
}

class Cat implements Greeting {
    @Override
    public void sayHello() {
        System.out.println("Greeting from Cat: Hello ");
    }
}

And the test code:

@Test
public void testGeneric() {
    Collection<? extends Greeting> animals;

    List<Dog> dogs = Arrays.asList(new Dog(), new Dog(), new Dog());
    List<Cat> cats = Arrays.asList(new Cat(), new Cat(), new Cat());

    animals = dogs;
    for(Greeting g: animals) g.sayHello();

    animals = cats;
    for(Greeting g: animals) g.sayHello();
}

Using “extends” in <T extends Comparable> is a promise that the data type will either itself directly implement Comparable, or else will extend a class implementing Comparable. You might have written a subclass B of another class A that implements Comparable, and if you declare your data type <T extends Comparable>, then you may use either A or B as your data type when instantiating the class.