Presumably because if they're not strictly kosher under US law, USD exchanges like Coinbase that try to stay in the good books of the US authorities won't want

Presumably because if they're not strictly kosher under US law, USD exchanges like Coinbase that try to stay in the good books of the US authorities won't want to touch Telegram's tokens for risk of legal action. It'd be pretty bad PR for Telegram as a company if the only way to buy their tokens was off shady unregulated exchanges.