I'd imagine even if there was no precedent for it, a good portion of Ferelden would be more favorable to it, on the basis that it's well, Elves, and are already at times in specific circumstances subjected to specific treatment.
That's different. Vaughn fouling his own nest by mistreating a few of the elves directly under his power is one thing, Arl Howe setting up a trade agreement to sell Fereldan citizens and labor to a much-reviled foreign country en masse is another.
At that level it becomes a political issue, potentially disrupting Ferelden's foreign relations(something neither Loghain nor Howe clearly give a figg about) by breaking diplomatic treatises with and offending all the neighboring countries who really really really don't like Tevinter or their practices. Orlais would be far more justified calling an Exalted March on Ferelden for fueling their enemy superpower with blood slaves than it would at the dwarves becoming more open for trade, for example.
The existence of the Alienages themselves implies that the elves are recognized as having some part to play in Ferelden's economy, and if anti-slavery is a strong part of the national identity then that would cause outrage all by itself. And I'm not very well-read on Andrastian lore, but given Andraste's war on the Tevinters and presumably slavery too, Andraste being Fereldan and Ferelden being a very Andrastian country, it might even be a religious offense. Which would also make it even more provoking to Orlais, the ultimate seat of the Chantry right next door who are itching for an excuse to reclaim their eastern neighbor.
Assuming that Ferelden politicians as a rule are more aware of the political landscape than Loghain, they have very good reason to be pissed. Arl Eamon being the one to spearhead the attack on Loghain's authority especially makes sense, him living practically on top of the Orlesian border and being married to the daughter of the Orlesian governor who previously occupied his lands.