Definitionally, exploitation is the action or fact of treating someone unfairly in order to benefit from their work. In other words, if a relationship is not mutually beneficial (fair) then it is exploitative.
Collectivists such as communists, socialists, and syndicalists criticise capitalists because they benefit (profit) from the production of others while they themselves are non-productive. They claim it is not mutually beneficial and is therefore exploitation. If this is true, then it is ironic that they advocate a replacement to capitalism where the non-productive benefit (profit) from the productive, that is something along the lines of: From each according to ability, to each according to need. If what occurs under capitalism is exploitation, then the same can be said of communism, socialism, and syndicalism. Alternatively, neither could be considered exploitation if one digs a bit further.
For capitalism, the capitalist is definitely productive in that they must have first laboured (invested their time, energy, and talents) to acquire the capital, invest it in a new enterprise, and employ workers to use the transformed capital in order to produce more capital. Capitalists also work continuously in managing the enterprise, overseeing management of human resources, marketing, distribution, and so on. In effect, the capitalist earns profit because it is their reward for taking risks with their capital, and because they themselves labour continuously to ensure the enterprise remains in business. The worker also benefits (profits) because they do not need to labour in order to acquire the capital to start the enterprise, nor are they responsible for its continued operation, they simply receive compensation for their labour. In effect, capitalism is mutually beneficial.
The collectivist critique of capitalists being exploitative in the sense that they are non-productive profiteers could be valid if the capitalist either acquired the start-up capital without honest labour (inheritance, theft, government intervention, etcetera) or they have no responsibility for the enterprise once it has begun operating (corporate shareholders). These examples are more applicable to corporatism, a bastardised version of capitalism currently in effect today, than to capitalism as it would exist in a freed market.
For communism/socialism/syndicalism, the non-productive are definitely exploiting the productive if the productive do not wish to share the benefits (end-result) of their labour. However, it is not exploitation if the productive willingly share the benefit of their labour with the non-productive (charity) so the critique becomes moot in this instance. The reason they would willing share the benefit of their labour with the non-productive is the same for any voluntary charity: They gain as much as they give. Some people feel good helping others, others see it as an investment in the betterment of society, and so on. In effect, communism/socialism/syndicalism is mutually beneficial.