"Subsidy" generally implies a third party transferring cash or cash equivalents to reduce or offset an expenditure. "Discount" or "break" generally implies that the party issuing the bill is voluntarily reducing the charge. The boundary between these is fuzzy, but I don't think it's right to call both "subsidies" and I do think there is a tendency for people to refer to discounts they don't like as "subsidies".

I think it's more coming from the perspective that taxation is the cost of doing business in an area. It pays for the training, infrastructure, security, etc that businesses rely on. Schools, roads, and police aren't free. Its coming from the perspective that governments pay the bill for companies operational expenses, and a company which is not pitching enough in to pay for these costs is getting subsidized by those that are.

I don't think "subsidy" is precise language but talking about "tax breaks" doesn't really communicate what a truly cushy deal Amazon is getting at the expense of others.