Lindsay Lohan -- Shopping Date w/ Star of Major Bust

Here's a question: if Lindsay Lohan really did cut all her toxic friends out of her life ... why was she spotted hanging out with a rehabbing hotel magnate who was busted for heroin in April?

Lindsay and pal Vikram Chatwal were photographed out in NYC Tuesday, reportedly shopping together ... you know, buddy stuff.

But here's the thing: Lindsay told friends she had permanently excised all the bad influences in her life in order to focus on sobriety and kick her substance abuse issues once and for all ... so it's a little surprising to see her hanging out with this guy.

FYI, Chatwal was busted in April after cops say he tried to get on a plane with cocaine, weed, heroin and various prescription pills. He struck a plea deal that let him avoid jail if he completed 12 months of in-patient rehab.

According to media reports, Chatwal's been to rehab AT LEAST 4 times since 2009.

We're not telling Lindsay who to be friends with ... we're just asking the question.