> Pretty sure it is legal to ask about your job

Maybe? It's a bit hard to say. You aren't allowed to discriminate based on source of income in Massachusetts. So if you filled out an application and the landlord emailed you saying, "oh, I don't want to rent to tech workers," it seems like they'd be toast since they're discriminating on the source of your income.

Yes, I think every rental application I've done has asked where I work. However, why should they be asking about something that they're not allowed to consider in their decision?

When you're interviewing someone for a job, there's a reason why you aren't supposed to ask certain questions. Even if it's legal to ask the question, it might not be legal to use the question in your decision. Why did you ask an irrelevant question? Did you ask because you're planning on using that information even though it's illegal to? It's not a great place to be.

As I said, I think every rental application I've done has asked where I work.

> and been background checked

I've never had this happen. I've had to submit a credit report, but that's it. Maybe the credit report seems like a background check?