I wonder why Slack so cares about blocking poor Crimeans from its service and at the same time Visa and MasterCard are accepted freely in any town of Crimea. Why these payment schemes are allowed to work there despite the embargo?
Why do you think so? Visa and MasterCard are banned in Crimea. Visa and MasterCard can't track its holder location.
Banned or not, there are ATMs and POSes across whole Crimea, accepting Visa and MasterCard. There was a brief period in 2014 when the schemes withdrew from the peninsula, but they get back pretty soon in 2015.