I wouldn't call it lazy, there are advantages to either method. Multiplication has the advantage that getting another 10% bonus is always 10% more of your *current* research, regardless of how many percent bonuses you already have - there's a nice mathematical consistency to it, and it's easy for a player to see exactly how much any *single* bonus will help you. But then things get a little less obvious if you want to figure out how much several bonuses together help you. Quick - how much total bonus do you get from multiplicative 15%, 20% and 40% bonuses - and is that better or worse than three 25% bonuses? If you have to reach for a calculator, you've already lost most players.

Adding on the other hand comes with a sort of built-in diminishing returns, which can be good for gameplay - adding another 10% when you're already at +100% isn't as noticeable as that first +10% was (it's really +5% to your current rate), so it lacks that mathematical consistency, but as a game designer you might want this if you don't want the numbers to get too out of control when many percents stack together. Adding is also a little simpler from a player comprehension perspective; two +10% bonuses are exactly the same thing as a single +20% bonus (not so if you multiply), this is simple and intuitive for any player. It's also nice mathematically that a 10% bonus and 10% penalty neatly cancel out additively, rather than leaving you with the awkward 99% you get from multiplication.

It's nearly the opposite when you get into negatives; adding is terrible here because the scaling is literally game breaking when you combine too many percent penalties, you can quickly get to 0 or even negative values, this is undesirable from pretty much any perspective. Multiplication scales nicely here, a 50% penalty is always half of what you have left, no matter how many 50% penalties you stack together, it can never hit 0 or negatives. Not too many negative percent bonuses in FE though, so I'd say going with adding wasn't a bad choice.