This is pretty clearly illegal. Under EU law, unfair contract terms are void/unenforceable, and clauses providing for one-sided modification of a contract without expressly providing valid reasons for such modification are provided as a quintessential example of such terms (see http://europa.eu/youreurope/citizens/consumers/unfair-treatment/unfair-contract-terms/index_en.htm). Someone should start a class action against HashFlare.