Yeah that's quite annoying when a healer does that. The threat system in this game works in such a way that every single HoT tick generates threat, which means that when the tank pulls a trash pack with let's say shield lob on one mob, all the other mobs linked to that mob will run to the healer because of a regen tick happening during the tank's GCD. This issue can be alleviated by 1. Using an AoE move to pull all the mobs at once if they're already close to each other before engaging the fight (Abyssal Drain instead of Unmend, Overpower instead of Tomahawk, but PLD aint got anything for this, they can just do CoS while the mobs are running to the healer, hoping that it will hit all of them) 2. Right-clicking off any HoT type buff before pulling (Regen, medica II, whispering dawn, diurnal aspected benefic, diurnal aspected helios) 3. Having the healer NOT cast a HoT before a pull. The most convenient way is obviously the latter, but most healers that are doing this either never played tank, or just have no idea how threat works in this game. I'll also add that any overhealing done (which means any heal done to a target that is already at max HP) has its threat doubled compared to an effective heal. So when a healer casts a regen before the tank pulls, the tank is most likely already at full HP, so the regen ticks are generating double the threat, for nothing. There is literally no reason for doing this other than pissing the tank off, and you were completely right when you told him to stop.
For bosses tho it shouldn't be a problem since it's all single target. Since you should be the first one engaging the fight with a ranged threat move (Unmend, Tomahawk, Shield Lob), you shouldn't have problems establishing aggro on a single target even with a regen ticking before the beginning of the fight (it's still useless for the healer to do that of course).